No doctor would ever diagnose a third party, not even face to face so no one here can diagnose on the forum!
I have many years of various experiences with mental health from various people and myself. Its such a wide are and the "norm" is so set in stone as being "perfect" that it would be deemed possible everyone has a mental health problem and OP, maybe your wanting to lable someone as far as to post their issue online could even buy a name for yourself! Most people let sleeping dogs lie...
If your worried about a friend, talk to them, if they are stressed about something (just because you don't see the stress does not mean there is no stress: its how we look at things that makes us stressed.) If they want to talk to you, listen, don't judge, be a mate and just be there to hold his/her hand. If he/she doesn't want to talk to you then you need to let them do their own thing. If you feel they need help, say something to them, if they do not believe they want or need any help then unfortunetly you cannot do anything other then watch them metaphoricly bash their head against the wall until they realise that there could be a better way to process things.
Just being there is all you can do if you want to be a good friend.