As has been said in the many, many threads on this topic over the years, there is no likelihood that a person with professional certification will have any liability for someone with whom he or she has a professional working relationship.
I use a lower level card, but not for liability. I have had multiple occasions when I used my card, the boat captain decided to pair me up with partner that was of a lesser ability so I could keep a eye on them, when they did not have enough staff on the boat. I was even told that to my face. Personally, if im not being paid to dive as a DM, I refuse to baby sit because the dive shop didnt have enough people show up at work that day.
I have have had a totally different experience. I
always show a professional card, and the result has
always been positive. I get put with the best dive groups when there is an option. I am always treated
very well and allowed to do things that others aren't allowed to do. I have great collegial chats with the operators. I have NEVER been asked to do what you described with another diver, and I am sure it is because they don't want me to be unhappy. They want me to recommend them to my students, and they want me to come back to them myself.
There was one time I was accidentally paired with a beginning diver. The boat was supposed to have nothing but pros on it, but someone in the office screwed up and put a beginner on board. I was the only pro who was not already paired, so he became my buddy. The operator was embarrassed and gave me my money back without my asking.
The only other time something like that happened came at the end of a week of diving. I was waiting at the shop for the next dive and chatting with the workers there, whom I had come to know pretty well. While we were talking a new problem arose. The dive I was waiting for was quite popular, and too many people signed up for it. For insurance purposes, they needed another professional in the water. I volunteered. I was given a couple of really minor responsibilities which did not interfere with my enjoyment of the dive, and I got the dive for free. I was mire than happy with that.