I'm more of a solo-diver, or as someone else put it "same ocean buddy." If they were to swim off and do their own thing, I might briefly look around before proceeding on my own dive-plan. I also tends to carry my own redundant air and equipment. What would I say to the buddy? Maybe ask then if they prefer solo diving.
In a sense, I feel it's not my responsibility to monitor, instruct, or baby-sit another diver or insta-buddy. There are many thousands of divers out there, and if I cared to make a difference, there are better ways than lecturing one diver at a time.
Now, if you're not ready for solo, then what I would do may not be that applicable to you. If you feel somewhat dependent on having a buddy to do a dive, and are with an insta-buddy, you're really gambling there. My personal philosophy is to never do a dive, unless I feel equipped (skills & equipment) to handle any emergency alone. I might pursue more challenging dive with a buddy I can trust, but never with an insta-buddy. Afterall, if your story, you were "solo-diving with a buddy." Because if you had an issue, your buddy wouldn't have been there for you.