She can dispute anything she wants. I'm assuming she's claiming the partial payment debit wasn't authorized. My guess is that she authorized it, or at least that's the claim the oil company will make.
If she received credit at the oil company toward her account, the transaction will not fit the definition of "unauthorized" in Regulation E.
Her gripe is with the oil company and you need not allow the bank to be put in the middle in this case.
John S. Burnett
Fighting for Compliance since 1976
Bankers' Threads User #8