My advice would be to point out that they need AOE threat generation to prevent the healer being targeted by enemies anyways, and DoTs don't require that much more flash/overpower spam. And to illustrate why it's incorrect that focus-firing makes clearing faster, you can point out that the DoTs are stronger than Ruin spam, so you want to keep them running on multiple mobs at once if possible.

But for a new player tank, it's going to be hard to understand that second point if they haven't had the chance to see the tooltips for themselves.