Dr., these are still Smiths speedometers so an error of 10mph isn't necessarily unexpected... even with the right speedo. Has your friend looked at the odometer reading compared to driving a measured distance? That will be a much better indication of whether it's the right speedo since the ODO is hard geared.
If you have access to the car or if your friend can perform a few basic tests you can determine what the Moke really needs. First write down the tire size. Next, jack up ONE front wheel. Put the car in neutral, pull the dizzy cap and spark plugs. Put a piece of tape on the elevated tire and on the ground just below that piece of tape (as pointers). Likewise, take note of exactly where the dizzy rotor is pointed. Put the car in 4th gear and have a friend SLOWLY turn the elevated wheel until you see the dizzy complete 10 full rotations. Your friend should be counting the number of wheel rotations. (There's a factor of "2" that comes into play here because only one wheel is turning)... but I can't remember if it's multiply or divide. You either take the wheel turns and multiply or divide by two (and move the decimal place one position because of 10 motor revolutions) and you should have a number between 3.1 and 3.6 Whatever the number is will be the final drive in the Moke. Armed with that and the tire size you can go to the Internet Mini Encyclopedia:
https://www.ime.org.uk/
Use the search field there for keywords like speedometer and/or final drive. There are tables there that tell you the turns per mile for various tire and final drive combinations.
I would also suggest your friend look at a different approach. Tell him to consider fitting a Sigma bicycle computer to the dash and not worry about the Smiths. The Sigma computers are well suited to automotive and motorcycle use. You bond or mount the wheel sensor magnet to the Mini driveshaft, make and install a mounting bracket for the speedo, and then extend the wires to allow placement of the readout inside the car. The Sigma units have very fine calibration scale factors to match the wheel size exactly and the Sigma computers allow programming the ODO reading to match what's on the Smiths. All this can be done for around $30 plus your time and sweat equity.