i've never ran into this problem yet...I've had 1 model long ago who wanted to bring her mom. It was tfp, i was new...so what. Her mom turned out to be a great deal of help. The girl sucked at modeling and her mom was more outspoken and not at risk of sounding rude than if I would have said what I kept thinking. lol!
Other than that, only twice has a model brought another person and it was usually a good lookin friend who I snapped a couple shots of too!! One paid shoot actually wound up being a double paid shoot for same amount of time.
I had a few cases where the model brought the mom, both were bad experiences. 1st one I had her ask me questions like,... "Why does my daughter need to be in a swim suit?" "What are these photos going to be used for" I was so pissed I was about to tell them to go home.
2nd time model brought the mom un-announced, everyone was pissed and the model was treated like an outcast. That ought to teach her to pull **** like that again.