Her only realistic option is to move. Since he's an ex-LEO, he will be able to find out where she moves to as long as it's somewhere in the US, so she'll either have to move out of the country or to someplace in the US where it doesn't matter that he knows she's there -- a state with very strong self-protection laws and no real restrictions on carry.
Even after she moves, she'll need to buy a firearm and get very proficient in its use.
If she stays here, the outcome will be either that she winds up dead or she winds up being a convicted murderer. Because the guy's an ex-LEO, the chance of the mom winning the resulting criminal proceedings are likely to be very slim.
No, much better for her to get out now, while she still can. She'll otherwise likely either wind up dead or wind up in prison with a life sentence.