One tenant moving out. What to do with last months rent?

[Matt Nolan]

Hello Bigger Pockets,

I have a rental property in Watertown MA, and the lease is expiring soon.  I sent out a lease renewal letter and I just heard back from the tenants this afternoon.  

The rental has three tenants consisting of a couple (boyfriend/girlfriend) and a mutual friend.  I was informed today that the friend would no longer be living at the rental and will be moving in with her boyfriend.

My question is this: How do I go about separating/refunding last months rent?  I currently have an account dedicated to last months rent ($1,850).  Do I refund the $1,850 to the three of them and have the remaining couple cut a new check?  Do I have the couple cut me an additional check for the new outstanding balance ($616.67)?  Is there another option I'm missing entirely?

I would think this is a fairly common occurrence, so I'm looking to the collective wisdom of BP to set me on the right track.

Thanks in advance,

-  Matt Nolan