This guy moved himself and his wife into her parents house from an apartment they were renting, thinking they would pay off some bills while with her parents, then get a home of their own. His wife now says she wants a divorice, their baby is 3 months old. I was reading if the father moves out of the house he may instantly ruin any chance for custody. He plans to only move out taking his baby with him, because his wife has said she doesn't want her, talked about killing her, using lethal injection on her , giving her away etc. But there is no proof. And the wife's parents will lie for what they want. The wife is calming down a little now about the baby thinking she may want her half the time, and My son would move out at the time it is agreed on is his time to have the baby.

Does that rule apply if it is the wife's parents home, and if he is worried for the babies safety.

There is no "rule" custody goes to who can prove in court they are a better parent or who proves the other is not.
If all things equal, most judges still gives custody to the women

What they need to do is get an attorney and file the correct motions in court.

Your son should try to work out a joint custody agreement with this girl in a neutral place, like a diner or something, but away from her parents and their home. Until there is an order, he has just as much right to remove the baby as his wife does to keep her. When my ex and I got divorced we couldn't agree whether it was night or day for a long time. So we waited before drawing up papers. We made a temporary deal that we would each have our daughter while the other worked, we split the cost of everything down the middle for any care or pre-schooling and we set up an overnight schedule that was flexible depending upon our work schedules. It wasn't always super smooth, but it helped. Even though I served him with custody papers that gave him less than 50% parenting rights, he was agreeable to signing them and for the most part things have run smoothly. Not everyone can be patient about filing though. This girl sounds a little off, so it would be in the child's best interest if the father filed right away for joint or primary custody if the wife is not willing to come to an agreement. Perhaps he should consider recording her if she threatens the life or health of their child.