When he is doing photojournalist style during the special moments, solemnisation, cake cut, stage toasting, march in, etc...
Never stand in front of the pro and block him.
Never stand too near behind him, if he back step he might bump into you and loss his shot.
Never follow the pro like the shadow, duplicating his shots, etc. He is going to get very defensive and may think people is trying to secretly learn his workflow.
When he is doing formals and portraits,
Don't also do and slow him down.
Go do candids or creatives instead.
Slip in for just one or two formal shot, when he takes a break to go toilet or smoke.
Slip out when he returns.
When he is doing table to table or group shots,
Never stand beside him and do also, to ensure everyone look at his camera instead other cameras.
I think if the second shooting friend/relative had done his homework and made it a point not to hamper the pro, and the pro still make fuss, then the problem probably lies with him.