You don't handle it! But the NH must. Pin someone in authority to the wall (metaphorically speaking, of course) and don't let them go until they've explained how they plan to resolve this.
Your poor mother. Reassure her at least that she is normal, anyone would be depressed and anxious if they were locked in to this situation. Hope it's sorted out very fast.
Speak to the charge RN on the floor and the social worker. Make sure they know that another persons behavior is affecting your Moms behavior. Ask them if there’s a plan to deal with the screaming. Hopefully they aren’t roommates, but even disturbances in the common areas can be upsetting to residents who have a lot of empathy. Make sure they are kept far apart at meals. You do not want this to begin to affect your Moms desire to eat in the dining room.
Another thought...what is the response from the staff when the screaming starts? I hope they don’t ignore it and let her yell? Someone should immediately come to her aid and try to calm her down and move her away from the other people. I feel badly for your Mom, but feel bad for the screamer too. I wouldn’t want to be inside her head.
Your poor mother. Reassure her at least that she is normal, anyone would be depressed and anxious if they were locked in to this situation. Hope it's sorted out very fast.