Honestly? It is none of your business who he dates or what he does now.
I have to constantly bite my tongue over what ex does. Constantly. He makes choices I would never do...but that isn't my problem any longer.
I only step in when it is something really, really stupid now. (like discussing moving in with his boyfriend (he is gay) in front of our kids without discussing it with me first. My 9 year old came home in tears saying she never wanted to move in with daddy and B because it is "so weird". I had no warning this was happening. My kids are 7 & 9 and my ex is stupid.)
It may hurt, you can see it isn't a good idea, but that doesn't make it your business. Unless it directly affects you or your children, then you just learn to roll your eyes and bite your tongue and move on.
Letting go is a long process. Stay focused on yourself and your future and let him mess up his own.