Most Helpful Girl
Drop her like a hot coal.
This is a sign to get out of the relationship now, while you can, without having to deal with something else happening later on. Tigers don't usually change their stripes.
Most Helpful Guy
Ask for her phone. Send the guy a text message. Something like...
"Listen, my boyfriend found out about us spending the night together. I feel really guilty now, you know, about what happened..."
And then wait for his reply. If it's "What? What happened?" or something then you can trust her. If it is something like "Oh no baby, has he broken up with you? Do you want me to come over? I never told anyone about what happened, so how did he know?" then you can't trust her.