Guides have a tendency to lapse into mindless repetitive patterns based what has produced in the past, and get surprisingly huffy whenever a 'sport' dares to suggest something different. Have seen this for casts, fly selection, leader configuration, clothing selection, etc.
Unlikely it had to do with spooking fish or boat stability unless your buddy likes to accompany his aerial cast with backflips or similar gymnastics. More likely the guide was concerned that you could not get a sufficient mend for the conditions from just his cast. Fishing from a drift boat is a bit different than wading, and mends have a tendency to be pretty aggressive which the guide usually adjusts for by maneuvering the boat. Not knowing anything about the conditions, that is about the only reason I could think of, other than the possibility that you hired one of those inflexible, conceited, Type A personalities that seem quite prevalent among guides.