Trouble is you cannot make someone else 'happy' even if you could help more, I mean I know in my mind no single person is going to make all my mental crap go away and make me happy and for me to expect them to would be cruel...

I think if you remain supportive towards this person and do your best to help as much as you can it would be appreciated, it isn't letting them down if you put in a genuine effort and simply just don't know what to do to help them or may not be able to do some things that might help. Even just being there for someone though can be helpful....Not sure what is going on with the individual you are trying to help but if it has to do with psychological issues, it is very possible they would need some professional help I mean most regular people don't know how to provide treatment for that.