Oh Florence! I am so happy for you in your new life and I am so sorry about this insanity with your mother.
Obviously detaching from her crazy-a$$ delusions is the only healthy, grown-up solution, and what I am going to propose is very manipulative, so take it with a grain of salt:
Maybe you could say "Mom, UMG (unmarried guy - your beau) is so good to me and my kids and we are just so fond of him. It really hurts his feelings to believe that you appear to like XAH better than him. Would you mind not telling people that because it keeps getting back to UMG and makes him so sad."