I would work here for free

Don't be so quick to judge.  People process grief in different ways.  He has been grieving losing her while she was sick, now he is getting through any way he can, and if that includes getting lots of sex, well ok, who are we to judge?  As long as he isn't leading these women on (and hopefully not involving his kids again, yuck) leave him be.  In a years time he will probably regret this behaviour, and you need to be there for those kids.