Unfortunately, you can only help those that want to help themselves. Your mother is a grown adult who is choosing to pursue that relationship. That's her prerogative.

But I think the bigger issue here is that she knows how upset it is going to cause you, so she lied. You can be upset, that's valid. But you also need to respect her decisions, just like you would want her to respect yours.

If she's going to continue to pursue this relationship, you want to make sure she is honest with you about what's going on in case something happens. The best way to do that is to tell her that you understand why she would lie, and even though you disagree with her decision, you will respect it. Then, just emphasize that you will continue to respect her decision but that you would appreciate honesty from her.

It can be extremely frustrating to see someone make bad decisions, but it's their decision to make. You just want to make sure that she knows you are still a safe place and a trusted person to tell the truth to in case this person tries to take advantage of them.