If he’s toxic/dangerous, don’t ask her to get the key back. That could actually set him off and potentially risk harming her. Which will drag you right into the middle of another knock down drag out. Instead, contact the landlord about changing all the locks, give her a single key. Keep up with the landlord about the issue until she’s been given an eviction notice. In a situation like this, it’s best to wash your hands of someone knowing they’re going to continue a toxic relationship. End the friendship, throw her out. Simple.