Well to be fair, if it's his first playthrough there's no way to know that in advance.
I recall that on my first playthrough I didn't bring the Cain along and had to take that thing out the hard way. Same deal with the Thresher Maw. Even that's not a guarantee though because on more than one occasion I managed to miss the shot with the Cain and totally wasted it...on the other hand I've also managed to take out the larval Reaper so fast Harbinger didn't even have a chance to show up.
Actually one of the things that bugged me about the heavy weapons in ME2 was that the game conditions you to hold off using them until you really need them so much that I would often forget to use them at all. First time round I think I mostly stuck with the rocket launcher as it seemed more useful than the others (more shots, less likely to miss etc.) To this day I can never figure out a way to properly use the cryo-freezy gun thing.