Look just tell her; I understand the boyfriend would get a bit unnerved, but that shouldn't be a reason not to say anything.  Besides, as long as you don't make it anything more than a confession, then the BF doesn't even need to know.  In fact, if you confess and don't talk about it until she breaks up with him, what's the problem with that?  Simple as that: confess and live with a clear conscious, and as long as you don't do something stupid, then he has no reason to be pissed at you, let alone even know you told her anything.