I´ve fainted during sessions before. I think it is due to the low blood pressure. First time I fainted, I got up from sitting down, and started walking. Apparently my eyes had started to roll to the back of my head as I got up, but I "woke" up several meters from where I was sitting. I woke up because the kambo was removed from my arm. Which is not a good idea. The consequence was that I was feeling horrible for almost ten hours, not able to stand or practically move. Throwing up every hour or so. I think the guy who applied it to me was very inexperienced, especially in this area. Something was just slightly off.
I recently went to another ceremony, where a guy fainted. This time with a more experienced practitioner, who simply held her hand on his forehead, and one arm on is back, leaning him slightly forward, and he was purging while being "unconscious", before he eventually came back to himself. Thereby still letting the kambo do its job, and not interrupting the process. Within an hour after application, he was good to go.
What happened to me during this ceremony, I was on the verge of fainting I guess. I remember embracing the "pain". I didn´t pass out, but what happened is that I lost my sight, and couldn´t see anything. Apparently I lost the color in my face, and was looking around the room disoriented. She placed her hand on my forehead, and I told her I lost my sight.
After a couple of minutes I was "back", though my vision was slightly disturbed for maybe 20 more minutes or so.
She also mentioned that fainting could be the bodies way of "escaping".. Whatever perspective you have on the reasons behind, I don´t think fainting is a bad thing, and obviously it´s possible to purge while unconscious.