I agree... maybe if you say, ok fine you pay for them then... it'll help her realize that it's not fair to make you guys pay for someone you don't even know. it's not like you're inviting a bunch of random people you've known only a few months, why should she.... it's not HER wedding!
I'm going to have to deal with something like this as well. my mom has a "BFF" and they have been friends for a year or two now, and I have been around her a few times... but she's VERY overwhelming and obnoxious, ESPECIALLY when there is alcohol involved... and I can seriously envision her taking over the entire reception and getting on everyone's nerves (she CRAVES attention even if it means picking fights with people and embarrassing herself and others). anyway, it's my mom's best friend and I'm sure they sit around talking about our wedding plans... so I feel like I HAVE to invite her... but I'd really rather not, and I am not sure how we could go about telling her... you can come but you have to be quiet and behave yourself... or better yet, DON'T DRINK!
anyway... sorry to tell my story in your thread. but I do think you have every right to refuse to invite these "friends" of your mother.