Fish do certainly feel pain but there's an important distinction to be made between pain and suffering and I think your friend is confusing the two. Suffering is harder to define but it could be defined as the ability to understand pain - if an animal can become depressed, can remember it's pain, can develop phobias, can grieve. It's less clear to what extent different fish can suffer. Despite how callous their reasoning may sound to an experienced vegan they actually sound like they're engaging morally with the issue and it's a great starting point for further reflection. I would encourage them to go pescetarian to start with and then gently introduce them to fish welfare and harmful fishing practices when they are more comfortable with a meat free diet.