What to do when one roommate moves out without notice?

[Lisa Craig]

I have a condo that I rent out.  I leased it in July for a one year lease to 2 friends who each co-signed the lease together as roommates.  They have not been getting along and one of the roommates just up and moved out recently with no notice to me or the other roommate.  Now I'm trying to figure out what needs to be done.  I have tried contacting the roommate who moved out with no luck, she won't return my phone calls or texts.

I have spoken with the roommate who is still in the condo and she wants to stay and is willing to work with me and pay the full rent or find another roommate (as long as I agree).  

I have several questions.  

1.  The lease has been broken because one roommate has moved out.  Do I need to create a new lease with the remaining roommate?  Do I have to somehow notify the roommate who moved out?  

2.  What do I do with the security deposit for the roommate who moved out?  Am I able to keep it for unpaid rent even if the other remaining roommate paid the full amount after she moved out?  Since the roommate that moved out did not notify me and is not returning my calls I have no way of knowing where she is to even return her part of the security deposit.

3.  I assume if the remaining tenant pays the full rent amount until the end of the lease term, she is the one who needs to go after the roommate for not paying her share, correct?  Do I need to do anything?

I have not rented to roommates before and am not sure exactly what is my responsibility and what is the remaining roommates responsibility.

Any advice is appreciated!

Thanks!