I think it depends on the situation and what was planned.. First I believe in the buddy process but I do solo dive also... When diving off most boats off LI, ny its not uncommon for several divers to go down together, and dive in the general area of others, viz is usually crappy and most don't put much effort in diving together.
If it was agreed upon that the buddy system was to be strictly followed, you should stick with the buddy as long as possible, if you are now putting your self in imminent danger, stop... A rescuer is useless if he/she has to be rescued also. In some extreme diving its agreed upon by the divers that after a certain point the diver in tbl would be left on his own. The big problem comes in when diving with long deco requirements or in a penetration dive... If you have a deco requirement you need atleast enough gas to complete some of your requirements.. If you have an extensive obligation going to the surface will probably have severe effects even death, but skipping small requirements will probably just entail a chamber ride.. You'll have to decide how much you are willing to jeopardize your safety.
On rec.scuba this thread is really getting lots of responses, one person brought up a death last summer of G.W., he had a problem (there are some conflicting reports, but it appeared to be a drysuit problem), told his fiancee to stay, she lived, he died.. If she helped most likely she would be dead also.. both were diving rebreathers, with a helium mix, they both had extensive deco obligations since the planned depth was like 300 feet. Blowing a deco on He is much worse than doing the same on air, He died several hours after the incident.
