Although it is ultimately the responsibility of each diver to monitor his/her own air supply, check gear, plan the dive, dive the plan, and maintain individual safety.... There is also a shared responsibility between dive buddies to watch out for each other and do buddy checks, share air (if necessary) etc...
By extension, there is a responsibility for paid (professional) guides to conduct a briefing on the dive site (including conditions such as currents, etc...) to assist in dive planning, to be aware of the abilities and limitations of all divers in their group, and to conduct the dive based on a matching of diver abilities and dive site conditions.
Although the paid guide may not be required to check each individual divers gear (although I would). And while it might not be possible for the guide to monitor each divers air throughout the dive... I believe that there is a moral and ethical responsibility that a guide has (as a paid professional) to the people in his/her group, for their safety and for the quality of the dive experience.
Hence, it is a "shared experience & responsibility" (between the divers and the guide).
After readiing about your experience, it sounds like that there was a break down somewhere along the line. Perhaps a not-so-clear understanding of the duties and responsibilities of every diver in the group (including & especially the professional guide).
Were you paying for a "private" guided tour or was it supposed to be a group tour? If the tour was supposed to be private and other divers were added, this should have been the "red flag" that something was not right. If this was the case, I would have made other arrangments with a different dive operator. (That’s just my opinion).
Thankfully, it sounds like the strongest link in your chain of events was between your buddy & yourself, and that may have saved you.
This story illustrates just how important having a good buddy team who follows good safe diving practice can be.
Safe Dives
Ron sends