Hi Diablo, A UK car driver with no motorcycle licence can only ride a 50cc scooter without taking any test. If a driver wants to have a larger CC bike they have to take a CBT ( compulsory basic training) test, this costs about £90 including hire of bike and limits the driver to 125cc. If the driver wants unlimited CC then they have to take another fast track access test which is quite expensive (£400+). After riding with a CBT for 2 years, you can have a larger bike but it has to be limited to 33BHP for 1 year.
If your friend has a full motorcycle licence then maybe he would be allowed to ride 125cc bikes on UK roads. I am not sure what maximum CC he would be allowed to ride, I will try to find out for you.