Should I add her fiancé to the lease?

[David Djamb]

 Hello everyone, 

My tenant has been a responsible tenant for 2 years now.  She always paid her rent on time and she is a low-maintenance tenant.  She asked me to add her fiancé to the lease.

What is my best interest?  I never met the bf, not background check done on him.

Should I just keep the lease the way it is and hold her accountable for everything or should I add her bf to the lease?

thank you for your help