Thanks all of you, for your support. It has been such a weird two days with this. She hasn't rang me or contacted me again after that.
I messaged her back this morning (before I had read all of your replies telling me not to!) saying: "Hi lucy. I understand that it must be hard for you to discover this out and I know you want all of the facts, but I have told you everything I know. As far as I'm concerned I didnt know he had a girlfriend, i thought he lived with his ex-girlfriend, we definitely didn't have ***, and it was just a stupid drunken thing. It was quite a while ago now and I don't even remember. I have told you all I know & I will stay away. Sorry about what happened but as I said, I honestly didnt even know you existed (a girlfriend, that is)."
She hasn't replied. I'm hoping she won't and will just let it go. fingers crossed I don't get phone calls late at night tonight...