If you've not experienced a control failure or propulsion failure in close quarters thinking through this situation and their response is a good thought exercise. It could prove to be valuable in future.
Thinking through their situation I would have first used the horn to get the attention of the yacht at risk and other boater's nearby, then the loud haiier to announce the problem. VHF would come last because the first consideration is the yacht they are drifting into. It is docked therefore they likely won't have the VHF on.
I will say having experienced control and propulsion failures in close quarters as deck crew and helms person each situation is different. Therefore the best possible response in each scenario is different.
The only actions that can be considered universally useful in any and all scenarios are to communicate the urgency, VHF, loud hailer and horn. And fenders, get the fenders out, as many as practical, as soon as possible.