I do, but I've only got the solid facts to work with. Don't want to go down as someone who spreads rumours. If I caught them making out I'd be telling the boyfriend for sure. Probably wait til he's back and tell him if she hasn't 'confessed' by then.
I would tell her to tell her boyfriend just what happened, otherwise you will.
If he is a good friend then you should obviously be telling him that his girlfriend's possibly sleeping about when he is not there.
But give her a chance to tell him first, and with the pressure of you possibly telling him she may just do that.
Then if you are really good friends with him, he will tell you about it later, and if doesn't match up to what you know then you drop a bombshell on him.
She wouldn't scream at you if she didn't feel guilty about what she had done. Her response to you is a direct acknowledgement of her sins. Your text was inadvisable, but she is wrong and she knows it. Fuck her for saying otherwise.
She seems to be one with no moral value, so I would not feel bad in the slightest if you were to tell her significant other about her actions, or if you were to perma-shun her from your life. She seems to be more trouble than she's worth.