I don’t think this is a thing that can be solved with breathing exercises. His heart and lungs are struggling to keep up with his excess weight. Weight lifting doesn’t really help with cardiac fitness. It’s good that he wants to strength train, but it’s unlikely to help his breathing.

He needs several things:

1. Dieting will help decrease the strain on his heart and lungs as his body mass decreased
2. He needs to specifically train cardio. Not running, obviously, but more walks. But you don’t want him to overexert himself. Instead of taking long walks, he needs to work on walking more often with breaks.
3. Being checked out by a doc is really important in case he has a heart issue like congestive heart failure from the stress he has put on his body.

I am very glad he is trying to better himself and wish him the best of luck.