If it is the local quarry, all the time, no problem. If it is one of my usual NC boat dives, then usually not a problem. Done many times. If it is one of our ledges and I have not been able to get out there for a while and the depth is 100 ft and it is a rank newbie diving an AL80 with air and they tell me they suck air and I have my usual HP100 and 32% nitrox, then I prefer not and would be very inconvenienced. The ops do allow solo. Probably would take him/her down and let them go up with somebody else prearranged depends on how comfortable they were and how nice the surface conditions are. When traveling and it is a special dive to me I just avoid the issue and hire a private DM. If it is just a keys reef dive we are back to the quarry attitude. By the way in a Keys reef dive that is a nice thing about a threesome on a boat that but does not allow solo. If you have an air hog you just figure eight back to the boat, see them up and then continue your dive.