Or he has a respiratory problem like asthma and needs to see a pulmonologist.

Exercise induced asthma is a thing.

https://www.mayoclinic.org/diseases-conditions/exercise-induced-asthma/symptoms-causes/syc-20372300

It is extremely difficult to exercise your way to weight loss. Getting your diet under control is much faster and less impactful for a person who is already 100+ pounds overweight. Once a baseline caloric deficit is reached and some initial weight loss started, it is much easier to get into some higher impact exercises and it will be much less difficult to breathe. In the meantime, walking is easy and burns calories and doesn't generally cause breathing issues.

Can he get to a pool?

At my pool in the morning and late at night several heavier people walk laps. They smile the entire time, and when it's just a few people in there, they seem to really be able to just relax and enjoy the exercise. Float a minute, or stand on the side and catch their breath.

I was at the gym with a guy several times a week that lost probabyly 100-150 lbs that way. Looked like a new person after a couple years.

He started just doing regular laps like me instead of walking around a year and a half though, he might have just liked swimming.

Middle aged fat guy here, but walk for exercise. Decent pace, about 5km per day. But one day, just after I started out, out of nowhere, I couldn't seem to get enough oxygen. Well, it felt like I could get a full breath, but nothing was happening with it.

I stopped until I could 'get my breath back', then continued my walk with no problems.

Next time I went to the doctor, I told him about it. Various tests, etc. Turned out to be Atrial Fibrillation. It's where the top half of your heart says, "Nah. Don't feel like pumping blood today. You can just have it trickle down to the lower chamber". Lower chamber fervently grabs what oxygenated blood it can to pump to the rest of the body.

Fortunately, able to be managed in my instance, with medication (though I did have a 'cardioversion', where they stop your heart for a beat to 'reset' it).

Given the severity it can take in others, I feel very fortunate mine is manageable.

Your friend might not have that, but they might. It's an easy couple of tests to find out.

I'm still fat, but I'm still walking, and more often than not, I realise how good feeling like I'm getting enough oxygen is. Which is a nice little boost.

https://www.lung.org/lung-health-diseases/wellness/breathing-exercises

I was a 3 sport athlete in high school and played trombone through college, so I never really had to think about breathing exercises as a standalone thing. My first question is how intense is the exercise? They may be overexerting during the workout, which causes problems with breathing. If the exercise is appropriate for their age and weight, then they definitely need to work on their breathing.

Most people, myself included, are usually pretty lazy with our breathing; we take mostly half breaths, punctuated with the occasional deeper breath. Both types are inappropriate for exercise. Tbh I'd probably start with breathing exercises suited for playing instruments; inhale through the nose, and allow the top of your belly to "expand" first. Then breathe out through your mouth.

This is your diaphragm, it's properly expanding in this method. Because your diaphragm probably hasn't done this in a long time, you might notice that your breath is shorter than your typical deep breath. This is normal. You aren't used to breathing properly, so you don't have proper lung capacity. The more it's worked on, the more automatic it becomes and improves endurance.

does he have asthma, or diabetes? sounds like asthmatic person. they need an inhaler when they are running, i knew someone hs like this.

Don't lose weight to get healthy, get healthy to lose weight.

95% of weight loss is diet not exercise. Your friend should consider a low carbohydrate diet, perhaps even straight keto. The weight will come down, including the ectopic airway fat that's impacting his breathing. His body will go through a period of recomposition, lung capacity will go up.

Angband already gave the best answer, a direct link to methods.

But, yes. There are multiple options for improving one's airflow. The first and most important is to get evaluated to rule out anything that needs to be directly treated.

Since there's already a link, I'm not going to waste time with that side of things. What I will waste time with is sending respect his way. Taking a multi pronged approach to improving health is the ideal way. But it's harder. It takes applying self discipline across every aspect at once. However, it also gives you cushion on each individual one because you don't have all your eggs in one basket. So, if a cold prevents breathing exercises, diet will still be working. If he has a day where ideal eating isn't possible, the exercise still improves other aspects.

And it is absolutely a fact that improving health isn't purely about excess fat. There's synergy involved with exercise, improved diet, and other lifestyle changes. Each one makes the rest easier and more effective.

The key is in not going too hard, too fast on any of them, except maybe breathing exercises, because there's really no drawbacks to pushing hard on those, within reason. But if you push your body too much in early stages of new exercise, you risk injury, and you'll end up burning out. Same with dietary changes; that's the mistake a lot of people make. They try to totally revamp their entire nutrition plan at once, and then wonder why they struggle. There's a learning curve to nutrition, and another curve to diet.

And yes, nutrition and diet can used as separate terms; nutrition being the understanding of what your body needs, and diet being what you put into it. See, even once you develop a deep knowledge of what you're supposed to eat, you have to implement it. People think it's as easy as just buying the right things, but it absolutely isn't.

Most people needing that kind of change also have built up metabolic dysfunction, built up tolerances to sugars and salts and fats and carbs (sometimes even protein, but I don't see that as often for some reason). You have to change those patterns, change the way your brain expects to feel after eating, change the way insulin is signalling you, etc. Changing that all at once, without any kind of plan for it is just not going to work well. And then people are left wondering why they either fail to notice improvements in health, or just give up because they're constantly stressed fighting their own body and brain.

So you have to approach that curve just as aware of the difficulties as you do when trying to learn anything. It's like climbing a rock wall. You have to choose your approach and think ahead of the step you're on.

So much respect for your neighbor, and anyone using a multifaceted approach to their health journey.

Tell him to keep at it, look into the breathing exercises linked. If walking for his main exercise becomes a problem, it's okay to switch to seated lower body exercise for a while. And don't let small setbacks blind him to the forward progress! It's easy to get frustrated when you have a bad day and think you're never going to get better. That's only true if you stop. Slowing down is still moving forward.