I really wouldn't read too much into this behaviour, it is a lot more common than you might think. Many children become aware of how their body reacts to things from a very young age, and there is nothing sinister involved. Many find these feelings more comforting than sexual.
I personally remember clearly behaving this way at around 6 or 7, I have a neice who moved like that on cushions, etc, from being a toddler, and one or 2 good friends whose daughters behave similarly. It is a little disconcerting if they do it in front of people, my sister told her daughter to do it in her room. The worst thing to do is react with shock, and berate or discipline the child for the behaviour. This will cause feelings of guilt and may affect how they feel when growing into puberty.
I'm sure your friend wants to rule out any question of abuse etc, this is only natural if she hasn't come across this kind of thing before. In itself, it isn't a sign of a child having been abused, it is quite a natural thing.
