Lawnmower is pretty peaceful fish to be in their with an aggressive trigger. If a grouper can't handle a trigger, you can bet a lawnmower can't. The other problem could be lack of food for the lawnmower. This fish frequesntly starve to death in tanks due to lack of appropriate food....real live algae.
Is the trigger's eyes completely glazed over, or do they appear cloudy from certain angles with eye movement?