was there a lease under

both tenants names ?

if so, then you will have sit down with both of parties trying to find a way to resolve the problem.

plus, didnt quite get the story

are you the house owner, aka, the landlord ?

or does your mom wants this lady to pay you guys back?

either way, it's probably the best for this lady friend to move out, and maybe you will just have to find another person to fulfill her place, in other words, you have to help her to sell her lease.