SIL at it again!!

Last week I had a call from someone who's girlfriend was "dating" my Son-in-law who is still married to my daughter. They have a son(11)and another one on the way. He also has 2 other children with other women. Obviously he has a sexual addiction. What robs me of my peace is that my daughter allows this to continue. $$$ is not an issue and she could walk out anytime. Anyone have any advice?