Tenant asks to move in a non-family member

[Ben H.]

New tenant asks if a non-family member can move in (friend of fiancée). Says he's willing to pay the application fee and have a background check.

1) I'm grateful she asked.

2) What are your thoughts? If mom or daughter wanted to move in, okay, that's a family member. But this a non-family member (which is different, at least in my perspective). Do I change the lease and up the rent? Charge them an additional fee ("non family member resident fee")? Run the background check and let it be?

Regards,

Ben