Are you responsible to help an insta-buddy descending to 50m? Honestly, it really depends on the circumstances, and there is no hard and fast answers, given so many different circumstances.
In this case, I dont think there is a correct response you can apply. You had your gear knocked off by the out of control buddy, IMO you would have been entirely justified in taking the "I wont become a 2nd casualty" approach. At the same time, it appears that you rescued your buddy without actually harming your own safety. Both conflicting answers to this issue can be true. (And I wont even mention if this had occurred in Malta).
As someone with only 10 dives, I would have recommended getting the guide to address the situation -- which is actually what you did, but in this case, the guide was completely useless. These are all changing circumstances that go into the answer to your question, such that there cannot be a clear answer.
Ultimately, you probably saved the life of a complete stranger. So no matter what, you should be applauded for your intentions and your execution, esp considering you only had 10 dives. IMO, you went above and beyond your responsibilities. I hope I would have done the same.
You should have been comped for that dive too!