I'm going to go against most of the advice here, and say tell her.
I was with my XH for 10 years, I didn't know until the end he'd cheated with 13 women (that I know of) One of them he had an affair 6month after we married and all through those years, one of the other women he slept with his is married to now.
I wish ....so badly one of those women would have let me know. The risk of diseases, lies, deception, for 10 years, when I finally did find out,
I packed all my belonging and my DC and Dpets and left.
He had the cheek to come up to my new place and say ...he didn't really want a divorce, just some time apart to get sleeping around out of his system..... WTF????
He had 10 years of our married to get it out of his system? My only regret is he wasted 10 years of my life. I can't get that time back, he had no respect for me, our DC, our vows, nothing.
I know as least for me, I would want to know. BUT You are wrong in the reason for wanting her to know, and personally I think you are both disgusting and vile for doing this to her and the DC.
But in the end I would have to know, I personally just couldn't take the lies, deception, and risk of STIs or worse.
My 2p.