If he has 7.50s and 205s then he will be getting some odd ball results for sure.
The differentials in the axles and gearbox are designed to allow the wheels to turn at different speeds. When you lock the centre diff (assuming you have a manual diff lock) you force the front and rear drive to be at the same speed.
Off road, this isn't much of a problem because the speed differences are lost when the wheels slip. Where there is good traction, you get a lot of stored energy in the drivetrain when you go round corners as the front and rear axles want to turn at different rates.
With severe wind up, you get a lot of strain put on the drivetrain and it can cause propshafts to fail, and in really bad cases can cause halfshaft or diff damage.
It is a very bad idea to mix tyres sizes on any vehicle that isn't designed for it, as you will get problems with the power and braking distribution, and most likley some oddball handling.
If the vehicle has a viscous diff and different size tyres, then you are waiting for something to fail.