I have a problem with my boyfriend...?

I have a boyfriend that I've been with for a little over 10 months now... we love each other so much and we never want to break up... I trust him, and he trusts me. Now, I'm not trying to sound like some sneak or some b**** who looks through her boyfriends stuff, he gave me his Facebook login, and he gave me permission to do whatever I want. So anyway, just yesterday he tells me he was talking to one of my best friends, and I became curious as to what about. So I went on his account and it was just all some normal chats, until around the end. She confesses to him that she's fallen in love with him (I knew it) and he says that he's sort of fallen for her too... Now may I remind you that he still loves me and we both don't want to break up. And my best friend talked to me about it and has apologized, but also admitted that she was trying to take him for herself. I don't know what to do... and please don't suggest I break up with him, or stop being friends with my friend... help please?