I've only once in my diving life had the experience of looking for an instabuddy. Two very nice people agreed to let me dive with them, knowing virtually nothing about me. I thought that was very generous. I didn't know that their habit was to dive completely without regard to one another (or anyone else) and the whole experience was horrible.
Ana, I think getting on a boat where the operator has told you he requires that people dive in buddy pairs (and therefore, anyone ELSE booking the boat expects to dive as a buddy) and then informing your instabuddy that you intend to execute what amounts to a solo dive, is very inconsiderate. I would be completely nonplussed by having someone tell me that, and I would politely excuse myself and go to the representative of the dive op and pitch a fit. Not every diver has the backbone I have; less experienced people may well blink and agree, on the assumption that this is common behavior, which it is NOT. (Now, whether your agreed-upon buddy, once in the water, behaves in any way differently from what you do, is another question.)
If I were to be on a boat by myself, and get paired up with someone with very different experience and skill (not to mention gas consumption), I'd be disappointed, but I would abide by the implied contract to behave as a dive buddy. Anything else is a form of default on a contract, in my opinion.