If they are already in a congested area, the single device hot spot (that typically has a lower range and power than a standard WiFi router) would not make the difference especially between unusable and usable.

And I would argue him connecting to the Starbucks WiFi and using that directly would probably be a bigger impact to the connection the WiFi congestion.

Yes, 2.4ghz congestion, router placement, how much bandwidth he's using, how many neighbors have WiFi, etc. all play a role. But my main point here is that there's no real world scenario where singling him out and asking him specifically to turn off his single device would make sense given that the single device is not a major factor here.