Tenant wanting to move out in middle their lease

[Sami Gren]

HI all,

I have tenants living with an older mother  (in her 60s) that has health conditions.

her daughter living with her, let me know today that she went to a doctor, and he said due to her health getting worse she should move to somewhere with less stairs, and she's moving as of December.

how would you go about it?

BEFORE YOU JUMP AT ME, LET ME EXPLAIN THE SITUATION. 

this tenant told me already a year ago that she wants to move out when her lease ends. When I gave her notice of rent renewel she said as she had mentioned all the time that she is not renewing because she's moving out, I even asked her a couple of times if she's sure that she will be able to find an apt. She said 100%.. and she even signed a paper that she will be moving out.

then.. very shortly before she was supposed to move out she suddenly told me that she is not moving and she gave an excuse that the reason is because she wouldn't be able to find an apartment due to her having a bad credit score....

now for this reason and others... I have an assumption that this  doctor story is just a way to be able to get of the lease....

my question is how would you deal with it in either case?

  thank you

then right before she