but their own reasoning doesn’t make much sense
Of course it doesn't. They're a non-vegan with empathy talking about animals. Why do you sound surprised?
Better for you to be happy and well-motivated to join in solidarity with other animals.
This is exactly one of the points why vegetarians and vegans are not “friends”. Abusing and exploiting animals is not really “joining in solidarity” with them.
Your friends are already evil. You are evil. I am evil. We all deliberately cause harm, knowingly or unknowingly. There is no ethical consumption under capitalism. If you can't be friends with evil people, you are either lonely or hypocritical.
One of the lighthouses in my activism is Daryl Davis, a black man who befriended Ku Klux Klansmen and got them to leave the Klan. People often stick to convictions out of community or habit, and living in a community where we accept each other's shortcomings and help each other become better is much more effective than any sort of moral condemnation. As long as everyone inside and outside the community can enforce their personal boundaries.
You're right that vegetarians aren't in solidarity with animals, but that doesn't mean we should never be in solidarity with them. Of course, don't have solidarity when a vegetarian makes animal product pancakes in a vegan kitchen; but when they do reduce their consumption, why not be their friend and cheer them on? You are better at it than them, but so what? Does a parent look down on their child for not being able to ride a bike?
Just be nice for fuck's sake.