I'd love for him to use a Mac with two monitors and wrestle with his dock constantly switching from one to the other with no way to lock the dock and keep sane window setups.
I’m not sure “the dock moves to the display where your mouse has hit the bottom of the screen” is quite the problem you’re making it out to be, compared with the founder of Microsoft calling their pride and joy a dumpster fire of usability.
I use a rMBP 2015 with two monitors (in addition to the laptop monitor) daily. One over HDMI and the second via Thunderbolt/DisplayPort to HDMI (so two cables into the rMBP).
I’ve never had this issue? What is your setup? I always thought it was funny that in a Mac you can’t set your default monitor, but I’ve never run into the issue you are describing.
I use a rMBP 2016 with one additional monitor over Thunderbolt to HDMI. I constantly have the same issue as OP.
Shouldn't you, though? Display arrangement lets you relocate the menu bar—imo that's as good a signal as any about which monitor should be the default.
Click on one monitor and then move mouse to another monitor and move mouse down to the edge (without clicking anywhere). Of all alien, obscure and mind bogingly bizzare ux behaviors this would be probably in my top 5.
I would hope that my dock would be on whichever screen my mouse is, so I don't have to drag my mouse across a large monitor to get way over there to get to the trash.
What you term "alien, obscure, and mind-bogglingly bizarre" is the only way I think should work.
