Your client is wrong ( sometimes you have to explain that to them) , semantically you should use tables for anything else BUT tabular data. and a menu is NOT tabular.
Also, a drop down menu is semantically a nested list, which conveniently allows for what you are trying to do since the sub menu are contained in the parent elements.
anyway, assuming your client is stubborn and insist on laying out his/her site with tables I would.. CHEAT
- code the menu in the form of a list and place it in ONE TD, leave the other one 'blank'
- wrap the menu in a div ( because TDs wont take position :relative ) , and give the DIV position relative the same dimensions as the cell.
- AP the UL ULs to appear to left:100%; and with the same height and width as the parent DIV
- tweak as need be.
BONUS: This is possibly even more robust than if what you were asking for were actually possible because if your content extends past the container it can overflow w/o breaking the whole layout. If this were possible to code with tables ( let's assume through the 'magic' of .js) if your content ever extended past the originally planed dimension the TDs would stretch and thus break the layout. Really tables for this= bad.
hope that helps