Hmm that's a tough one. Maybe if you get a chance to talk to her alone again, you could just come out and ask her if she would be more comfortable being monogamous with him. What she needs is important too.
Trying to confirm he said/she said rumors is probably not a good idea. It could lead to them having a fight...and the one who's story you were trying to confirm to mistrust you. That really wouldn't help things move along at all