ok... not all disabilities are PHYSICAL. sigh... so while you may have had to help your dad fill in a form regarding him doing physical things... some people have OTHER DISABILITIES that are perhaps not physical. Again, you are letting your frustration with your friend's situation cloud the issue.
You have no idea what type of disability he has, so you have no idea if he is claiming fraudulently. If it's for mental illness of some sort, and you stir up trouble unnecessarily, do you think that's going to help him in any way? Of course not. But hey, you judge hitch your judgey pants up a bit higher and wade through the muck, no problem.
You have literally NOTHING to base this accusation on other than the fact that you see him around doing things and that you don't like him and you think he's a bad parent. As you have no clue regarding why he is claiming disability, you have no clue if the things he is doing are contradictory to his condition. So what if he's playing tennis? Unless he's told them he can't walk, it's really not a big deal. You know, people with mental illness are allowed to play tennis and do physical things - it's actually encouraged. Yes, it's not nice that he isn't helping his ex more, but let's face it - that's hardly uncommon or against the law, is it? It certainly doesn't add up to fraud. And while you might think he should be trying his best to get a job, if he has some sort of mental illness, perhaps his physician is better placed to know whether or not he is ready to work or not. Unless, of course, you know his complete medical history and you have a medical degree.... no? yeah, thought not.
Honestly, if you spent as much effort in helping your friend find support and assistance as you do nosing about in this guy's business, you'd be much better off.