Cheater?

So I met a guy I really like and we hit it off. We have a lot of similar interests and he has been nothing but kind to me. I told my friend about it and his current girlfriend interrupted to say that she knew my guy and he had cheated on her a month ago. He sounded really confused and said that he didn't know what I was talking about. I don't know what to do about it because she doesn't seem like a very reliable source but I haven't known him long enough to automatically take his word... What should I do?