My mother is getting a roommate at her assisted living facility. How do I prepare her?

My mother is acting like a child at her ALF, she takes about other residents in a negative way (always been like that) and talks loud so everyone can hear her. We have to have a talk about that tomorrow, not sure how or what to say. She will get angry and demand to leave facility. She will also be informed that she will be getting a roommate real soon, not looking forward to this at all. Any thoughts?