I think people are missing the most important thing.
There are two parts to this. That the OP is stressed and upset that she wasn't there to comfort her daughter. Natural, understandable and how we feel and the strong emotional reactions and protective reactions we have as adults towards our children can't always be predicted and aren't always 'rational'.
However, her DP took the 6mo into bed when 'he knows she's starting moving about/rolling about in bed'. So really, he shouldn't have done that in the first place because it SOUNDS like it's not the usual sleeping place for DC and therefore hasn't been planned and isn't a planned co-sleeping arrangement - hence why DC rolled off the bloody bed!
So would I be angry with DH if that happened? Yes, 1. for the rolling off the bed incident itself and 2. that would mostly for the fact that it was entirely avoidable and shouldn't have happened.
Unplanned co-sleeping is dangerous, planned isn't as demonstrated here.
However, people do make mistakes. I put my DS aged about 3 months on the sofa and he fell off - my own angst at that glaring error wouldn't have needed a telling off from anyone but had someone been there to witness it, I'd have had to accept the bollocking because it would have been justified.
DP seems to somewhat be slightly minimising what happened and to possibly not see the level of danger and what COULD have happened potentially (ie he could have rolled on top of DC), he sounds a bit blase (but that could be me reaching, I'm just responding to how I felt and the thoughts I had when I read the original post).
Getting mad angry at the fella probably won't help, understanding why you felt so strongly would be useful for the OP - was it the potential and the possibly poor decision-making that led to the child being put in bed with him that has rattled you? Then a calm discussion with him about co-sleeping needing to be properly planned and the format of it agreed to by both of you not carried out ad hoc.