Tenant in studio basment apartment wants to add boyfriend

[Will Morrison]

Hi, 

House hacker here. I've got a tenant who wants to add her boyfriend to basement studio apt. It is currently only her on the lease. I am not opposed to allowing him to move in down there, however want to protect myself as the land lord. My question is, what is the best way to go about adding a tenant to an already established lease? My first thought is to have him fill out a Zillow application (to better understand him as a tenant) then make an addendum to the lease that maintains the primary tenants responsibility however allows the boyfriend to stay there. Ideally, making her responsible for him. What are the thoughts?