What's the best way to change the lease when 1 tenant moves out?

[Galen Bricker]

I recently rented a single family home to a couple who checked out well.  Unfortunately, they broke up and she moved out and they still have 15 months of the lease remaining.  He has a buddy that wants to move in with him to help with the rent.  Right now, she is still on the lease.  How do I handle this situation?  Do I write up a new lease?  Do I need to get something in writing from her to drop her from the lease?  I've never had this situation and wasn't sure what the best method for handling this is.  Thanks for your help!