I've been there, it's difficult. Best advice I can offer is don't let it linger, and don't make it official if possibly avoided. An off the record / closed door talk with the Chief Pilot (or most sensible minded senior manager available) might be the way ahead, who can always then engineer a way to find out for his/her self.
What happens then will be out of her hands, but either disciplinary action if it's gone too far, or more hopefully some for of "sort yourself out" programme if it's not and the bloke's worth keeping. But pax aircraft with a short bottle-to-throttle time can't be tolerated and has to be knocked on the head before somebody gets hurt.
And yes, it is everybody's job. The art is to sort these things out without hurting people if humanly possible.
G