Whilst I agree that 'any old pilot' in the other seat of a single crew operation could quite likely be a liability, the two Pilots in this case were both highly experienced Corporate Crew who had flow together professionally for a long time, understood CRM and had a wealth of knowledge between them, even if one was fixed not rotary.
Having flown single pilot ops in complex types many years ago I did appreciate a competent pilots assistant as they used to be termed. As long as they are briefed/trained in the scope of their input I have only seen it as a good thing.