This really depends on how close of a friend we are talking about. I’m not suggested hiding anything as such, but this is an issue of modern day dating that I like to call recycle dating. Due to the speed at which we are now dating and the fact that many dating apps are linked into a specific location, it’s only natural that you will date a guy that your friend has dated or vice versa. You could have also dated or hooked up with a guy and then realised you have also been with a friend of his.
But do you tell? If they are a close friend yes because if this relationship continues, you will feel like you have lied or will be forced to continually keep a secret. If you then tell your friend or they find out, they might feel betrayed because you kept this secret in the first place. If you are going to tell, ensure not to tell too many details. Your friend needs to make up her own mind about this guy (unless he is some abusive psycho).
If you do want to tell you could always use a bit of humour. After all this can be a rather funny situation and something you or your friend might have experienced before. Why not make a quick humorous comment and not a big deal out of it.