It's difficult. obviously if you were to intervene (for example by pretending to take an interest) to help, the scammer would realize what you are doing.

you are saying that your mother is not senile, and is just (acting) 'stupid'. But if this is the case, why don't you ask her about the details (excuses and so forth)?

Has she told you things like, "Yes, I realize that there are scams, but this is not one of them."

Is she otherwise intelligent? If she is intelligent and you got as far as her saying, "Yes, I realize there are scams, but this is not oen of them" then you could discuss rationally how you know...

the scammer could also kind of believe it from their end.