Simplify the following text for a Grade 9 native English-speaking person.
Original: After years of internment , like Brainin and Nissel , Schidlof became a pupil of Max Rostal , and it was Rostal who became the mentor of the Amadeus and persuaded Schidlof , who had been a violinist , to become a viola player .
Simplified: When they were freed they became pupils of Max Rostal who persuaded Schidlof to play the viola instead of the violin .