You know he has done before so you know he is capable. At the end of the day is finding out whether he did have relations to her or not going to help build this relationship back or make it easier for you to leave?
Some advice Im not sure how you are going to find out, but, if you are going to ask her, try and do it by text so its written down, that way if she does say yes then you can try and recollect dates and times. She has nothing to lose and no reason not to be honest, having said that she could be bitter that the "relationship" is over..

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