Add tenant's dad to the lease?

[Pier C.]

I have a great tenant who's been at the house for 1 1/2 years and is now on a M2M. I recently discovered that his dad has moved in temporarily, though I don't know for how long, after recovering from what I gather was a rough spot. My lease states that anyone residing on the property for more than 2 weeks must be added to the lease. I'm willing to make an exception here since my tenant is responsible and is clearly the one paying all the bills, but would I be making a mistake? How important is it that dad be added to the lease?