Yes Willy should have been coming back full tilt to cover Werenski, instead he was just sort of drifting. But OK, if it becomes a 2on1 then Gardiner played the 2on1 very poorly. Instead of staying roughly in the middle of the two attackers as he should, he drifts all the way over to Panarin as he's approaching the blue line, this over-committing early leaves Werenski wide open, then as the pass is made, he immediately starts drifting over towards Werenski leaving Panarin wide open.
I believe different coaches will say different things here and it also depends on a few things (the angles and "handedness" of the attacking players probably being the main ones) so there's no easy 100% clear cut answer here but I think most would say something like this: You don't want to just stop the pass and let the guy with the puck have a breakaway, you want to stay roughly in the middle of the two so you're not giving the puck carrier a clear path down the middle and at the same time, you're making it difficult to make a pass. A full description of strategy would be a lot longer than this but I believe that's the general idea.