Ah so they had sex during the first three months, but she didn't enjoy it because it seemed forced from his side and then they stopped, I see.
Their problem seems to be a clear lack of communication/honesty, from the very beginning; I mean, when sex isn't enjoyable, couples are supposed to talk about it and find ways to 'spice things up' and not keeping quiet about it and just stopping sex altogether.
From what you said, outside the bedroom things seem to be going alright, they love being with each other, have a great time together... but they need to be more honest with each other if their relationship is ever to work. This is true between friends, let alone boyfriend and girlfriend. How long did they think their relationship could last if they avoided tackling a big issue like this one?
I didn't mean you should talk to him about his problem, but rather encourage him to open up with his (ex)girlfriend. He needs to do it for his own sake; whatever his problem might be, it's obviously a big issue; he can't expect to ever get into a serious and successful relationship with anyone if he can't be open and honest about it. If he really loves your friend, he needs to trust her; sure, talking to a professional could be an option too, but opening up to those who love you us always bound to bring better results.
As I said, part of the reason why he might be so reluctant to open might be because he doesn't exactly know what's wrong himself, but he must learn to share his doubts, insecurities... he needs to let go of his fears and let people, namely his girlfriend help him. Everyone needs help at some point or another, it's no shame; you should try and make him realize that letting his girlfriend in can only be beneficial for him.