Do DFVCP now. The fee to file is small enough that it shouldn't even be a question. Admit the mistake to the client and explain that you are going to eat the costs since it was your mistake. Consider yourself lucky that you caught it before the client did, or worse yet, the IRS or DOL.
The rule is that it has to be for repair of damage to the participant's principal residence due to a casualty loss that would be deductible under sec. 165, but without regard to whether it was located within a federally declared disaster area. The situation you described sounds to me like it would qualify but I am not an expert on section 165.