The same thing that happens to them if you *don't* leave the church...nothing. They do their pretend baptisms for dead people. It means nothing. It means nothing if you stay, it means nothing if you leave. All it does is keep people coming to the glorious temples...giving them something to do there, and keeping them paying tithing.
Baptisms for the dead never really made sense to me. What happens when the priesthood guyou mispronounced the name, which happens all the time? Or a name that two people have? Does God just automatically know who it's intended for? That must get confusing really fast. What happens when the same person gets baptised twice? Does it cancel out the other?
As long as you can psych someone into believing you're "worthy" when you perform the ordinance, it stands.
My chidren's father baptized our oldest daughter. Then a few months later confessed to several affairs. He was put on probation so couldn't baptize our 2nd daughter the next year. I always thought it was funny that God will accept an ordinance that was performed by someone who had sinned, lied, not confessed. But an ordinance performed by someone who had confessed and repented would not be acceptable because he hadn't done enough groveling time.