If he is asked to declare medical conditions then, while its not illegal to do so, it won't do him much good to have lied on his application. It is illegal to discriminate against people on the grounds of disability so your friend might be helped by that, however my experience is that if you don't want to employ someone, there are legal ways not to.
If he isn't asked, he neednt tell; but if he needs regular work time off for checkups, again its going to come out....