Changing the lease to a new co-tenant?

[NA B.]

My fairly new renter who has a company paid stipend for the rental home expenses wants to remove her husband and add her new boyfriend to the lease. (I told her he needed to be added if over 18). Now wondering if I truly want this new, possibly non-financially capable person to really be on the lease. Is it required? What if she leaves town to next company move (stated as a possibility) and he stays and doesn't pay rent. Looking for good advice please & thank you! 

*I used Zillow for her and her husbands background checks/lease signing so now I can't get another background check run on new guy...