When a bike ride goes all wrong...
There are rules of the road. But in this incident, none of them would have done much good.
What do you do? Having a plan helps a little. First, it is a good thing this rider has a companion. Normally, we say, riders should go in a group of 3. If one gets hurt, one stays with the injured party, and one goes for help.
Having a basic first aid kit can be helpful. If you tumble, you want to do everything possible to let the bike precede you. Otherwise, in addition to the rough ride down, you could have a heavy bike impale you at the speed of a freight train.
I would rather fall off my bike and down a cliff than stay safe and bored in an office.