Some attendant might get scared or something and create a scene of that such.. and even from your explanation the man might not even be mentally unstable but probably broke and ratcheted. Not everyone who looks dirty and unkept are mental. Some are just fine but suffering from depression. Literally I don't see any reason why they shouldn't be allowed to gamble. If they have money and can place the bet then why not. Being mental doesn't actually remove ones right from reality, it one removes you from the public as every one sees you not fit to live with. So if he can still gamble then he shouldn't be deprived of a chance to.