Not to pick nits but critical engine has to do with if there is an inoperative engine it can lead to loss of directional control. 337 cannot have a critical engine nor for that matter does a seminole or duchess due to counterrotating props.

As far as how airlines perceive 337 time, interviewers who know anything about general aviation aircraft will certainly notice it and be curious about it. They will wonder if this 337 time is to avoid a normal multiengine airplane, which of course it is not in this case. It seems unlikely the time would be a minus as long as he has a normal AMEL ticket. It will help in the total time column, but will be worth little in the ME column to a knowledgable interviewer. Therefore he should still get some normal ME time and sprinkle it in to show he can fly both, and be prepared for that possibility of not getting full credit for all the 337 hours as ME time. It's hard to say what he will have to do if this happens because they may say he needs more real ME time, and he should be prepared for that. If he puts at least a certain percentage of it as seminole time it will be purely window dressing since seminoles do not have critical engines, but it will help and I would do that. Just giving my thoughts... I am not a hiring person.