I would make a copy of the bill and send it to her every month.
Bills are bills, we have to pay them. (Mother love is not good sometimes).
If your neighbor or best friend was having this problem, wouldn't you think the bottom line is, "this is not good for the child to learn, that they can treat people badly and not pay back loans etc.
Cyna, I feel your heart break. (I have listened to lots of mothers, and it is never the money)
Hopefully you can let it go for a while to give you time to think, then you still have to go with your gut.
Just my thoughts, I don't know what I would do.
((Hugs))