They didn't start charging more. Telecom companies pull some shady tricks with plans and billing, but increasing the monthly fee without notice part way through your contract is not one of them.
He signed a contract for some period of time, and it wasn't up yet. After it was up, he would have had the option of one of the new plans, or switching providers, just like everyone else does.
With mobile providers in the US, if you don't use your "upgrade" you can keep using the service with no contract. If you accept their upgrade, an offer of getting a new phone at discount, you lock yourself in for another year.
He signed a contract for some period of time, and it wasn't up yet. After it was up, he would have had the option of one of the new plans, or switching providers, just like everyone else does.