Security Deposit Return/Roommates

[Remmy Vernon]

Had a young couple move-in to one of our apartments. She bailed after 3 months. Luckily, he was able to find a roommate. ( he went through our application process etc. and was passed all criteria)

I use a lease that states 'All tenants if more than one, are jointly and severally liable for the full amount of any payments due under this agreement'.

I am trying to figure out what to do about the SD. My gut tells me I should not refund her half the deposit back. I feel like the SD stays with the lease. Is that correct?

On another note, she asked me for her half, and I told her that she should contact her EX and new roommate and that I felt they would make things right with her. Which I think they will. I only want to get involved if I have too. IE: force the new tenant to pay half, then give her half back. 

What have you guys done in the past in this situation? I want to be fair to everyone involved.