Well, long term health issues and facing our own mortality is a pretty common cause of depression. There's a huge connection between physical health and mental health, as we all know .
Change is really hard and you can't force it; most people have a lot of cognitive "distortions" about the way things are in their lives. While I think it's great to offer the use of your 3 wheeler to gently move things along, I wouldn't expect that there is going to be a miraculous change in attitude. There's a lot more at stake here, too. He most likely is depressed, but if he is really operating with 50% lung capacity and a "weak heart," (what exactly does that mean? congestive heart failure, blockages?) I would want to know if it's OK for him to even attempt to ride the trike. It sounds like he is a candidate for a medically supervised weight loss/cardiac rehab program that includes nutrition, counseling, and exercise. Most major medical centers have this, as do some community hospitals. There's only so much friends can do and it sounds like his health issues are serious.
Frankly, I have 2 friends that I've known for years; both have told me point blank they have given up. They just expect to gain weight each year as they age and one, especially, feels she "can't" do anything, no time, nothing works. One used to be extremely active and sits and talks about what she used to do. She had knee surgery last summer and is back to tennis, but she basically talks about what she "can't" do. She hides the belly fat pretty well, but she carries herself like an old lady. She's my age. My other friend has always been a bit heavy and never been an exerciser. She's really just hung it up, as far as healthy living. Her husband had a 99% blockage in one of his arteries, had a stent put in, lost some weight and now is bigger than ever; he is huge. This was 2 years after my DH (who is fit and thin) went through his cardiac issues, so it's not like they don't know how important exercise is. Her DH can barely walk without breathing hard.
I gave up.