Let us know if checking off the diagnostic allows you to e-file without a problem.

It's not clear from your post whether there is a refund due on the return.  What we know is that neither you nor your client are surprised by this level of withholding.  Theoretically, unless your client has a substantial amount of outside income (relative to his/her compensation) plus the compensation is much higher than the §911 exclusions, assuming he/she is in a low/no tax jurisdiction, having over 50% withheld would seem excessive, taking into account the stacking rules.  Depending on the actual outcome, I would still suggest that you revisit the W-4 to make the appropriate adjustments.

The fact that your client has actual US tax withheld tells me your client probably moved to a low/no tax jurisdiction with no local withholding requirements and assignment allowances/benefits may be minimal.  Otherwise, tax equalization or protection may come into play, not the least, to address cash flow concerns - sometimes, only after the close of filing - and needs to be accounted for when the reconciliation takes place.