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.