My friend told me after I found she started dating another girl at my school, but I had to ask her. I asked my friend 2 months into my relationship if she had any feelings for my girlfriend and she lied telling me " I would never do that to you". I think my girlfriend knew, but she won't give me a straight answer.
Anonymous

If your girlfriend knew and didn’t tell you that is where the issue truly lies… If your friend never acted on her feelings then her not telling you until later was just to keep you confident in your relationship with your girlfriend.

I’m trying to look at it in different ways, but if I were in your situation I would be livid. I wouldn’t do anything drastic though, but you should definitely let them know how you feel about the situation.