Did he just order and take delivery of this car in the past couple of months? For a short time earlier this year, they offered Standard Range Model S and Model X which were battery limited to 92% of the 100kWh battery and cost $8000 less. 335 miles*0.92 = 308 miles for the S, pretty close to the observed 310 miles your neighbor is getting. Before any were delivered, they eliminated the Standard Range option for the X but kept the Standard Range S around a couple additional weeks. Sounds like he might have received the Standard Range S. They don't have new badges yet so they put the 75 badge. Our new Standard Range X was updated before delivery to be Long Range and has the 100D badge even though it shows as Long Range online. Does his show as Standard Range?
If he checks his battery, he will probably see the part number is for a 100 kWh battery. Option code should be BTX6.
So just had a 75D loaner car from enterprise. Car had 260 miles range when I got it @ around 90%. I charged it to max trip mode and it got 272 miles range. I figured it was a November 2018 build since it showed Fremont Supercharger $0.00 - November 2018 on charging screen. Sounds like there are a few unicorns out there with bigger batteries. Didn't check if ideal range was set though.