What do you think about telling my significant other to not let her mother borrow money from her?
Why would you want to tell your significant other not to help her mother?
I don't want my significant other to get scammed by her mother.
If your so's mother is untrustworthy it is up to your significant other to do something about it. Telling her what to do is interfering in their relationship. Warning her about it should be fine though.
If she keeps letting her mother scam her, I'm going to be pissed. It'll likely end up ending our relationship.
It's really unfortunate that it's gotten to this point but it's not your place to tell her what to do. All you can do is ask her to be careful and hope she does the best with that information. Telling her what to do will just make you seem controlling.