When puchasing a 2-4-1 through Plat/Cent Amex, you essentially pay the price of one full J but get two I class tickets. Hence, if the OP and his friend travelled in their original class (J) then they would both get J miles. The companion tix therefore is treated as a revenue tix, not an award/free tix.

Now that they paid a flat upgrade fee the miles they get depends on what the check-in agent enter onto the check-in system. Everytime an agent issues a boarding pass they will have to confirm the ticket class code be it F, A, J, Y, B, H, T, whatever. That's why sometimes (very rarely though) one would get miles even on a non miles accrual ticket, such as in the case of an award tix - the agent enters F/A instead of Z or J instead of U, by mistake.