Ugh. Well, this is terrible and frightening.
They both need help.
I would talk to Nick first. Tell him you are concerned that his mom is having a tough time and is taking it out on him. Try and gauge how bad it is if you are lucky enough to get him to open up. Suggest counseling at school if that is a possibility. Tell him you are going to talk with his mom . Tell Nick you are there to help. Explain that he can call you if she hits him or harms him again.
Talk with his mom. Tell her you heard that she slapped Nick.Tell her you can’t imagine the pressures she is under. Tell her you care for them both and offer to help her get help. Let her know that you care for them and you feel a need to make sure she doesn’t hit Nick again.
There has to be programs for parents whose kids have disabilities. Look on line. Make phone calls. Gather information anonymously . Your goal is to find them both help if they need it. Ask on the thread for parents with kids with disabilities.
The issue with MD is that it only gets worse. You can’t leave him to handle this alone.
Give your daughter a hug. Tell her you have got this. Tell her she did the right thing by telling you.
Good luck.