Is it love? Was it love?

I'd like to ask for advice about something that happened to my best friend lately.
Look, in february she got a boyfriend. They got along so well the first time.. But now, she doesn't know what happened, he doesn't talk to her anymore.
The strange thing is, it wasn't a progress or somethin like that.. it happened from one day to the other. Last saturday they were talking alone, they were happy and such. They didn't discuss. And since Monday she hasn't seen him or talked to him, he seems to avoid her completely.
I've been thinking about it and I don't know what could be happening.. Some people think he maybe did something he shouldn't have done and is avoiding her because he doesn't want to tell her. Well, knowing him as I do I don't think so, though it may be one of the only logic explanations..
I don't know what to say to her, which advice to give her - what do you think? I would really appreciate some comments..

Thank you.