If you like him, you should continue dating him.
Recycle dating (where we date someone who is an ex of someone we know or are friends with) is common these days thanks to the speed at which we are seeing and dating different people. It’s bound to happen especially if you are dating in the same area as your friends. It will depend on how serious your date and your friend were and how close your friendship is. If they just dated, then you might not need to initially say anything at all. You need to make up your own mind about him without judgments from others before you open up those gates. Your friend might say things that happened between them or negative things that could sway your mind and be based on her opinions not yours.
If you do have a close friendship, then you might want to make light of it to her and have a bit of a giggle about it. You don’t need to ask for her permission if they were not in some serious relationships but ensure you are still respectful and also place boundaries if you don’t want to know details of what happened between them.