My partner childhood dog got killed by his mothers new pet, what can we do?

A few days ago, my partner‘s childhood pet, a chihuahua, got killed by his mother’s new dog, (It’s important to note that my partner still lives with his mother as we’re both young, and can’t afford our own place yet) he was coming in the back door, and his mothers dog, who she’s had for 5 months now, got overwhelmed, clamped down on the chihuahua, causing it to have a seizure and then die. My partner is obviously distraught, as he’s had that dog for a long time, and his mothers new dog, a mix of far too many breeds to be called a cross, has only been in his house six months. However, we don’t know what to do, as his mother is insistent on keeping her dog, even though there is two more small dogs in the house, and the big dog has shown similar aggressive behaviours toward them, and the big dog has had to have had her mouth pulled off the smaller dogs heads before. Is there anything we can do that will result in my partner and his pets feeling safe in their home? I’d made this post earlier today but I’d left out some information, so I’m reuploading