They keep charging you the original rate presumably, which now only gives you X TB of storage, not unlimited, and as he did not move to increase the amount of storage his plan has (by paying more), he was essentially underpaying his bill the entire time

I’m not sure what sort of pricing he would have with Enterprise (it’s “call for quote”), but the cheapest published way to get the 250TB or so of cloud storage he needs would be to pay $900/mo for a Business Plus plan with 50 users