If your husband is a reasonable man, tell him! He has a right to know what is going on in his home and at least you are sharing the burden (obviously if he is hot headed then don’t, it will probably make things worse).
I’m trying so hard to think what I would do if it was my son in this situation. I think I would go to the police, notify them what has been going on, son scared, threats to him and herself and that you want to ask her to leave and ask their advice. Maybe they would recommend them being there, maybe not, I don’t know but at least they will then be aware if she kicks off.
As far as I see it the worst thing that could happen is that my son would leave with her. He may be crying out for you to deal with this for him but is afraid to ask, or he may be angry that you’ve broken his confidence. Honestly in this situation I would lie and say “your dad caught me crying and I told him everything and he is insisting that she leaves”. Your DH can’t be unaware of the situation to some degree? I hope faced with both his parents concern he would agree to accepting some help?
practically, I would get your son to take her out for lunch and pack up all of her stuff. When they return don’t let her in, tell her she is leaving and that you have booked her into a Travelodge or give her £500 and tell her to go, that you cannot take the atmosphere in your home created by her and she is no longer welcome. Tell her you have advice from the police and if she doesn’t leave quietly and leave you all alone they will be notified. If possible keep your son in the house and away from her. I have sympathy if she has mental health problems but you can’t let her destroy your son or your home in the process.
And unless she is a good and responsible dog owner, pays for their food, takes them for walks and to the vet, I would keep them too.
I don’t know if this is good advice but honestly I was trying to put myself in your shoes and I’m bloody furious and frightened for you . Good luck x