First off not all groups tend to have a problem where people start to complain, some just force themselves through if they experience hardships.
My experience about complaining people in groups is, that often the weakest person is going to complain the fastest, once it knows that the whole trip is too much for them and that they don't know whether they can make it. The person can feel that it is the weakest and that alone demoralises it a lot. It's lamenting it's own weakness and my advice for you would do something with your gf that makes her stronger.
You can observe it well when you go with families mountaineering. Usually the youngest/weakest child starts complaining first. I have been in that role often, being much younger than my siblings and my father not being able to distill a better discipline in me.
Sometimes however I was on a trip with other children, and they started to complain much faster than I did. However I didn't join them doing so. Instead I tried to help the other person and help the group spirit, even when I was at the point of where I usually would have started complaining. I have seen them phenomenon in different situations where people would swallow their complains and naturally shift in a role where they would support the group when another person starts complaining - even if they felt the same and where close before bursting out.