Lets pretend for a minute that you're me.

Why are you pretending this is a big 'scary' job when it's not?

When my handyman and myself arrived they pretended to not be home. 

Pretend you thought the boxes belonged to one of the trades people working on the house.Even if he does try to sue, the name on the lease is NOT his.

As for figuring out the rental rate in your area..... there are "services" like rentometer etc...... but I fond the best way is to just pretend to be a tenant looking in that area.....go to all the websites that a renter would look....

Pretend you're looking for a place to rent. 

I’m about to house hack another 4 plex, I think pretending not to be the owner is more complicated than it’s worth.

Or do I let it slide and pretend like I never saw it?

There are a lot of people with alterior motives of stuff to sell you, many that have no idea what they are talking about though they love to pretend, and many that give advice that may be appropriate for them and their market but don't apply to you and yours (real estate investing is local and personalized).

You would be making a big mistake letting her stay, people never change they only pretend to change to con their landlords.