My friend does not enjoy visiting his daughter because of this specific situation and she usually has to pay out of her own pocket for a hotel.
that is her problem, not yours.
Basically she keeps stating that her husband thinks it would be a good idea to bring his daughter here and hang out with her while my friend and I do our thing.
I'd be tempted to give her husband $5 and tell him to pick up his daughter and take her
to a coffee shop for the day.
Seriously, there must be things they can do together that don't involve your home. Museums, parks, galleries ...
When my parents visited me while I was in university, we were usually so busy with activities - farmers markets/picnics/walks/plays etc etc that I was exhausted when they left. Our hanging out time was always crazy active.
I can understand that you would like to have time with your friend but she needs to sort out the dynamics in her new family.
She can book a hotel. Hubby and his daughter can visit there. You can pick her up for a spa afternoon and take her back to the hotel. They can have the rest of their time together as a family.