I have found that more often than not, usually people appreciate it if you inform them of things that might be off-putting to other people. Especially if you know the person and has regular contact with that person, like a friend.
Just don't be weird about it, be overly informative about it, or try to get too involved in his situation.
If he has an issue, he knows about it and will tell you, and if he needs help he will ask. Other than that, give the person a heads-up and leave it at that.
Well, this has been my experience.