Ex wife trying to ruin my boyfriend

First things first, my boyfriend(34) is currently going through the divorce process with his ex(29) they have one kid(6).

So my boyfriends BM is constantly complaining about anything and everything that has to do with me which I understand and can handle. Unfortunately my BF is now worried that BM is going to try and fight for full custody (Currently it's 50/50) she non stop threatens it. If he doesn't do as she says she threatens it. We barely make it paycheck to paycheck so paying the court cost is killing us already. The most recent time she threatened it was when she saw me picking up my BFs son from the bus stop (it's a 1 minute walk from our house). I don't know what i can do because my BF doesn't want to lose his kid. There's no reason a court would take him from my BF as he's a good person along with being a police officer. We have tried to stay civil with her as we can't afford to fight her in court (her mom is paying her court cost) we have evidence of her actively screaming at the 6yo and video of her lying in A court document. It is starting to take a toll on my BF mental health.

What can we do? any advice welcome.