Is your client still legally employed by his/her US employer and seconded overseas for an assignment?  If your client was transferred or locally employed, your client should have no W-2 while working overseas.  Even if your client remained a legal employee of the US employer, exemption would normally be claimed for FIT, which means there would only be FICA withholding.

It would be highly unusual to have FIT exceed 50% of Box 1, particularly for someone who qualifies for foreign earned income exclusion and considered neither the withholding tables nor supplemental rate would get the FIT anywhere near that level.

Something is not adding up...