The captain is wrong. There is no standard that forbids this.
That's not to say that there couldn't be liability issues if something went wrong. Lawyers are masters of making non-issues sound important. In terms of bail out, the rebreather diver may even have more options diving with OC divers than they would have while diving with other RB divers. There is an issue, however, of the fact that this diver probably had a buddy assigned by the dive team that in theory should have done a buddy check but wouldn't be qualified to pick up on possible issues related to the gear. For example, would the buddy have been aware of limitations to descent speed and what the ramifications could be if they descended too quickly? In the case of a law suit following an accident, a lawyer would make that kind of thing into a big deal and the dive team could end up twisting in the wind.
So no. No standard but in my mind there could still be an issue. The most prudent thing to do would be to buddy up the RB diver with another diver with a similar level of training (even if that diver wasn't wearing a RB).
R..
