^^^ All I really need to know is they took off. I get there may be other factors that led to this collision but there are no factors to excuse the abrupt departure in my world.
Apologies to keep chipping away at this, but again, we don't know the circumstances that led to their actions, so no, that is not all we need to know to pass judgement.
It appears that the Dad was ahead of the kid when the incident happened. Collisions are a frequent event at a ski resort, and it's possible that the kid did not know the extent of what happened. Remember, he was not injured in the collision, so that impact was not significant. The lady presumably suffered all her damage form hitting the tree, which the kid may not have been aware of.
So, the kid rides up to the Dad and this might be the exchange;
Dad "where have you been"
Kid "some lady and I hit each other on the catwalk"
Dad "what happened"
Kid "I joined the catwalk from the trail and she was just stopped in the middle. I couldn't avoid her and she fell into the trees"
Dad "are you OK?"
Kid "I think so"
Then the angry husband rides up;
Husband "your kid just knocked my wife over"
Kid' I'm sorry, I didn't mean it"
Dad "is your wife OK"
Husband " I don't know, I decided to chase your kid down first"
Dad "it sounds like everyone is OK and he said he was sorry."
Husband "well, tell him to be more careful next time"
I know I'm filling in a ton of blanks here, but so are you guys, and my scenario is just a plausible as yours. Remember, at this time, neither the Dad, the Kid nor the Husband knew the extent of her injuries. If they had, maybe they would have stayed, but thinking this was just another case of two skiers bumping into each other, did what we would all do, and went about their day.