While I sympathise with the new player and agree that there should be a better tutorial or some small safeguards in casual CC to help the newer players, the monk is neither toxic or wrong, pressuring the backline is literally their job as a melee, also WHM can usually be a priority target because of their kit.

As for how to counter that kind of thing, generally pinging a quick chat of "Attacking monk!" will usually wake some of your teammates up to look for the monk and realise that their teammate is being pressured.