This is a very difficult situation! I'm sorry that it happened, and I'm glad that you are no longer with this man. My first thought was that you have to tell her. I would want to know if it were me, because you cannot heal something until you know how broken it is.
You need to pray a lot about this before making a decision one way or another. People have referenced scripture from both sides, so I don't need to do that, but Phillippians 4:6: Be anxious for nothing, but in everything, by prayer and petition, with thanksgiving, present your requests to God.
Proverbs 3:6: In all your ways acknowledge Him, And He will make your paths straight.
There are a lot more, but the point is, He wants us to come to Him with these things before we decide, and before we do them.
Keep in mind also, that there is an extremely good chance that her reaction to this isn't going to be very good. She is not likely to be outwardly thankful immediately upon hearing this. I am not saying this to discourage you from telling her. Simply to remind you that when you do, make sure you bring your grace, mercy, and a thick skin. You will likely need all three.