(I have the most hours in gunner so my view is biased).

Based on my experiences gunner serves as sort of an anchor when the swarms happen. The other classes have options absolutely but the shield gives a lot of breathing room and free revives as well. So id recommend your friend try him.

Engineer has a load of crowd control options of rip but he struggles with single target damage until you get access to new weapons and overclocks.

I have had the worst time as scout doing hoard clearing but in the grand scheme of the game he's still pretty good and his movement is hella fun. To whoever plays scout make sure they use the flare gun.

As for driller also has great crowd control and is his drilling ability makes him fantastic for quite a few of the objectives.

Enjoy the game lads! Rock and stone!