I'm thinking of possibly ending one of my mother's friendships
Why do you want to end one of her friendships?
Her friend is always talking about boring things.
Your mother may very much value this friendship and ruining it over something petty is wrong.
I am doing my mother a favor with this.
I do not think you would be doing her a favor. Let your mother end the friendship if that is her wish. Do not try and do that for her, she may still like her friend.