IF you were charged $300, it is because you were still under contract with verizon. Either his phone was recently upgraded (5-6 months ago?) or someone else used the upgrade on another line.
Read your contract, under my Verizon, My documents. When you purchase a subsidized phone, you agree to a monthly line fee for 2 years.
FYi: I believe you could have done a transfer of billing responsibility for his line without incurring an early termination fee.