I have been there on this one! So here are my thoughts on this...and after I will tell you exactly what happened to me so you can understand....
He is going to tell you what ever he thinks you want to hear and what ever he knows will make you happy and calm you down in order to make things right....you wont' know the truth until you call her...don't text her, call her. Ask her what's going on and what he told her at the same time introduce yourself as his girlfriend. Dont' attack her because it isn't her fault and there is no need for it but talk to her nicely and explain the deal. After you have asked all you ask and find out all you need to know confront him.
I say this because I was once told this and found it to be the key to solving issues like this...it's quite obvious that I have beeen with not one but 2 cheaters in my life and they were both serious relationships- "Never confront anyone until you have all of the facts and can leave them speechless" I swear by it and I don't accuse or confront until I have found out all I need to know. This was actually advice from my first boyfriend's brother.
Now I will tell you that the first time I actually ever used that advice (because it is so hard to do really) was when I was 22 years old, 6 years after I was actually given the advice. It worked out perfectly..no things werent great but I got my answers and there were no games and it solved the problem fast and I left him speechless...
Here is what happened...my boyfriend (now husband...this was years ago and he has learned since trust me) was cheating on me and I had a feeling. The "witch" even called and I picked up the phone...he told me she was a friend from high school that he was trying to hook up with a friend...I found other signs of things lots of them...she lived in another state...and he went to visit her....supposedly visiting family...anyway there were a lot of signs and things just didn't add up. Well one day not one but 2 people he knew kind of told me about him being with this girl and going to see her...that was it. I actually had her number and at that point found it was the last straw and I needed to find out what was going on...so I called her. I played dumb and told her that the number was on my bill and asked who she was, then I asked if she was a family member of my bf because it would only make sense that he would call there...she really was dumbfounded...she knew about me because we had a child together but didn't know that we were together, she wouldn't share information about their relationship but I told her by saying that she wouldn't share the info she just told me all I needed to know b/c if there was nothing going on then she would have just said it. She really didn't seem to care that he was with me at that point and decided she would stay with him and thats when things got rocky with she and I and then of course he and I. I hung up with her and called him and he said "Hey baby" my response was "hey baby? Sure you got the right girl on the phone or are you talking about your witch?" Well he was speechless....didn't know what to say, I basically told him the jig was up and I know it all and that it was over and I hope he was happy and that she was worth it. That was that I got my answers. We split up and they lasted about a year until she bad mouthed me and he left her anyway....but my point of this whole thing is that in the past so many times he had cheated and I didn't have all of the information I would run to him and question him about it and cry to him and tell him that he will never be able to make kids again if he did it and he told me all I wanted to hear when I knew that he was lying to my face...I couldn't do anything abou it b/c I didn't have all of the facts and confronted him before having them so I had no real grounds and he won...this time was so different because he was completely caught.
We actually split for 2 years over that and had nothing to do with each other what so ever. He realized what he lost and actually admitted it all to me years later, every girl, everything he did, and it hurt but I told him I knew all along...
So do I think you should call her? Absolutely, just to find out the truth and let her know you exist. If he gets mad tell him that he gave you no reason to believe him, and if he was lying then who cares if he gets mad he was wrong.
Then just remember for next time with anyone else always find out your facts first because they all do the same thing in telling you what you want to hear.