Have you checked the TAB file itself? Right click on the saved .TAB file and open in Notepad or another basic text editor and see what the version number it has saved in the top is. It should look something like this.
If it says
!version1050 then it might be worth changing that to
!version850 (make sure you make a copy!) and see if that works.
Failing that you could export it as a Mid/Mif file and see if your client can import that instead.
Normally when I get these problems it is because I have used a Date/Time field which are not compatible with older versions, but since you say you only have Char fields this should not be a problem.
The Geometry might also be a problem if it happens to be really complex object types (multi-collections, text and so on) might struggle in older versions, although I doubt it, but you never know.