If you splash with a buddy you are a not a solo diver, or in the event of your buddy having an emergency are you are swimming away alone as it has nothing to do with solo diving? Although one team member is less skilled than another may put more responsibility on the other, it does not make him a solo diver. Some solo skills may come in handy, hence the PADI self-reliant diver specialty, but there is still another diver involved.
Sorry but this is one of my pet peeves when discussing solo diving. Solo diving is done alone. Buddy diving is with another, even if your buddy is a danger to himself and others, you ar not diving alone, you may be in a worse position than diving solo.
Bob