No Emily you're reading it wrong I think. Robert says he thinks Brian could've PREVENTED John from getting addicted. Which I don't agree with. John wouldn't have listened to Brian anyway and perhaps Brian himself would've gotten addicted too, who knows.
However, I agree with the Buffy quote SatanHimself posted. The group broke itself up. There were a lot of factors that made them want to broke up, but like I said, they WANTED to break up. Hadn't Yoko been there, they still would've broken up anyway.
I don't agree on Yoko having a positive influence on the Beatles. On John maybe, ...