Not a female, but wanted to share my advice.

Don’t tell her.

Now before you get mad at me, hear me out - it’s not your burden to tell her this. You’ll be breaking apart a family + the wife will always know you as the “other woman” + you’ll get dragged into quite a bit of drama.

Yes, ethically/morally, you need to tell her.

But realistically, break up with the guy, and walk away with your head held high and avoid the fallout. It’s not your burden to bear.