In my opinion, a paying customer should never be asked to be a guide unless it's an offer of employment, meaning that instead of paying for the trip, you get paid. Since I travel alone, and never know before boarding the boat if there will be another single to buddy with, I make sure there will be a DM who I can hang with. The sort of diving I do, there always is.
I am a better snorkeler than diver. Once on a week-long stay at a place with a variety of organized water activities, the guide put me with a person who had never snorkeled before, was not especially comfortable in the water, and was not physically fit, and basically ordered me to take responsibility for her. I did it, but I resented it, because I was paying to be there, not being paid to work, I have no rescue training, and being assigned to baby-sit an inexperienced person meant I could not do what I wanted to, which was get some exercise by swimming faster than she was able and do some shallow freediving. Several times, the guide came over and said "Where's your buddy?" as if to say that he did not trust me to keep an eye on her after I had reluctantly agreed to do so. (She was following immediately behind me, and I knew exactly where she was and what she was doing as I was keeping a careful eye on her.)
When you pay to dive (or snorkel) I think you have a right to enjoy yourself, and should not be asked to baby-sit someone whose skill level prevents you from engaging in the activities you had planned.
In the OP's case, I think the owner had an obligation to tell the clients, at the time of booking, that there would be no DM and they would be on their own in the water, and that if they wanted a guide they should book with an operator who provides a DM.