I've done the trip with a solo diver along. It was no problem. He did some diving while others snorkeled and some rendevous. If the diver took 15 minutes longer than the group, no one cared at all. Some advice - not just for diving, but in general, especially with 10 onboard, everyone needs to make an effort to be tolerant of each other, flexible, and considerate.

I'm now a diver myself. I rarely ever dive alone, but he's an adult, right?