Tenants break up and want to break lease

[Nicole S.]

I have a couple (aka girlfriend & boyfriend) who signed a 1 yr lease about 4 months ago.  They have since broken up and decided the girlfriend will stay and they will find a tenant  to move in and take the boyfriend's place.  

They have found a tenant that I have screened.  I am ok with her other than she has a low credit score (based on little credit history and high student loans from what I can tell, she is very young and currently lives at home).

My question is, should I assign the boyfriend's interest in the lease to the new tenant or somehow add the new tenant to the existing lease in order to keep the boyfriend on the hook should the new tenant not pay.

Appreciate any advice.