For liability, it seems like an absolutely massive stretch to make a customer responsible for essentially baby-sitting another customer or employee. Their certification level is absolutely irrelevant. Even rescue-diver cert is really a courtesy to other divers, and I'd probably prefer to just skip that cert if it did cause a notable increase in liability.
Absolbloodyutely
A friend of mine was asked to buddy with another diver a few weeks ago. He is an instructor and was planning a solo dive. He agreed provided that he did not have to pay for the boat trip.
If he's not diving with one of his friends then like me, he dives solo and is appropriately prepared for it.