he is 28.5, she is 27 they are together 18 month.
i thought about everything all of u said but maybe he has a problem and he is shy about it?
Ok, next question: why did they suddenly stop having sex? I mean, you said they did in the first three months of their relationship but it lasted 18 months, that makes it 15 sexless months. Did the guy tell her after three months he was asexual, did it just happen naturally and the relevation came after your friend couldn't take it anymore or...?
Either way, just like in any relationship or friendship, honesty is the key here. Any relationship is doomed from the very start if both parties can't be honest with each other. If you want to help them, convince the guy to open up with your friend somehow. If they truly love each other and feel their relationship might be worth saving, the only solution is full disclosure and honesty. They need to talk about it, as they found out there's only so long you can avoid such big issues in a relationship, they eventually come back to haunt you.
I'd say there's a very good chance the guy himself doesn't know what exactly his problem is, he's probably scarred and confused and frustrated, that's normal. But if he can't trust his 18-month girlfriend with whom he apparently had a great relationship to help him through this, who can he trust? He needs to let go of his pride (if that is even the issue) and open up, it never helps to keep your problems to yourself, especially when there are people willing to offer help and support.
The way I see it, the only real solution to this situation is to encourage both of them (namely the dude) to improve their communication, be fully honest with each other... Regardless of what their problem is, if they do love each other, there's probably a solution. It is indeed hard for a relationship to survive without sex, but it is even harder, actually impossible, for it to survive if both parties can't be honest with each other and fully trust each other, sex is the least of their problems.
Just my two cents.