They are obliged to tell you.
If he had a 50% NCB, full bonus, then his premium will double in year 1, with 10% reduction in year 2, 20% year 3, 30% year 4, 40% year 5 and 50% year 6.
If he had a protected bonus, it depends on the degree of penalty, reduce from 50% to 20% or to 30% depending on insurer. Some insurers have fully protected ncb which means that there should be no affect on ncb.
The statement that 'it wont affect ncb, but he will be loaded' does not make sense.