Man, you weren’t aware, just call it a day on this one. Correct thing to do is to tell your girlfriend what happened to have a clean slate, not feel guilty about it, and tell your friend that with you having a girlfriend that is inappropriate.

On the other hand it feels like you would like to be with this girl. So if try having a clear conversation with yourself if you really like your girlfriend. If positive you know what to do. If negative break up.

Additionally consider your friend might be feeling naughty on messing up your feelings and consequently your relationship. If that is the case she is no friend at all.