Is This Legal?

I’m asking this on behalf of a friend bc I’m very skeptical; So my friend’s mom lived with her daughter, son in law, and their children to help them get back on their feet financially. The mother ended up moving out due to her & her daughter having a major falling out.

The daughter & her husband moved a new person in who is NOT on the lease & the rental company was not made aware of this new tenant. This new tenant of theirs is a felon, to add if that changes anything here. Meaning no credit check, application, or background check were done.

The mom has been paying her half of the rent since she moved out 3 months ago because some of her belongings are still at the house that she couldn’t move on her own. Now that she is prepared to move her things out before March 1st when rent is due, the rental company has given her some information about breaking the lease. The rental company told the mom that she was legally obligated to stay on the lease and continue paying rent monthly, even if she is fully moved out, UNLESS the daughter & her husband agree to break it. Which we’re assuming they won’t because it saves them money. Is this legal?