I have only abandoned my buddy (there was no group at the time) one time. It was to be a simple deep dive to look at the attractions with a max depth of 100'. Whether this makes me a bad buddy or not, is in the eye of the beholder; I did what I felt I had to do, to ensure my safety. Perhaps we should have had covered the plan of what we would actually be doing down there in more detail. We dropped down & started the dive. My buddy decided at that time to start penetrating some of the cabin cruisers & such down there. We were both AOW divers with deep, but not wreck experience. I was not at all comfortable with the idea of entering an overhead environment at such depths & so would just hang outside of the attraction & wait. Well, as my air started to get low, I started signaling my buddy, but he decided to ignore me. When my air supply reached a level where I needed to start an ascent to be able to surface with right at 500psi . I signaled my buddy again that it was time to end the dive. Once again he ignored me & continued to go inside the attractions (he had a pony bottle, I did not). I went ahead & began my ascent & when he finally surfaced I asked him what was the deal down there. He claimed that because he had the pony that he wanted to push the limits of his dive (obviously we should have also discussed at what pressure to begin to ascend). He also let me know at that point that he was teaching himself solo diving & that he figured I'd be OK if I went ahead & ascended before him. So in effect I was basically just a buddy to let him get into the water (the quarry will only let certified solo divers dive solo) & all others must have buddies to enter. In that case I feel I was justified in going ahead to begin my ascent when he ignored my signals to end the dive. Needless to say, I will no longer dive with this diver as he wants to be a self- taught solo diver & how he finds a way to get into the water (finding a buddy) is his problem. Besides this episode, I have seen multiple things this diver has done that I would consider dangerous considering his training (we go through the same shop, so I'm aware of his skill level). I will also quietly warn new divers to the quarry about him & his general M.O. Unfortunately this is a character that if he doesn't wind up killing himself, he'll probably wind up killing someone else. Case in point, after diving a rebreather only 1 time, he offered to teach me how to use a rebreather. Of course, I know my limits & I'm no where near ready for something like that. I will only take larger steps like that under the watchful eye of my instructor, who is qualified to teach such things. Was I a bad buddy with this guy? I don't feel like I was, but then many will come to their own conclusions.