We use a .dev domain as a localhost alias, and turns out his ISP’s DNS wouldn’t resolve 127.0.0.1 (or whatever it is) for the .dev domain. Changing his resolver at the network level to 1.1.1.1 fixed it.
I imagine there are lots of difficult support tickets for app devs, and at a certain point they just hardcode the DNS to remove one variable from the equation when debugging bug reports.