I think Stu hits it dead on. I am a software guy and so (by extension) I can appreciate the frustration you go through on a regular basis at work. Who needs that at home when you should be relaxing. We have all had projects that just won't seem to end (currently working on one myself) or nothing seems to go right but if there is not even a glimmer of joy or relaxation in you even during those trying times then it just might not be the hobby for you. It may even turn out to be an unsafe one if you push things or let your anger and frustration get the best of you - only takes a split second loss of focus.

Stu's suggestion of taking a rest is also a good one - allows you to reconnect with what got you into it in the first place. If after a month you don't miss it, then perhaps it is not for you.

BTW: I am in the same boat as your friend. I love music and have tried to play on numerous ocassions and just seem genetically incapable of getting it. Same goes for foreign languages.