1. A tableless layout is great, but there are some pitfalls:
a. Automatic patches most likely won't work anymore.
b. Installing third party mods can become a chore, as the code won't correspond to their install instructions. Most of them still use tables
c. X-Cart add-ons won't work correctly out of the box
Basically you are creating a lot of extra work for yourself down the road until Qualiteam and all of the developers follow suit. You need to weigh the benefits of that yourself. For me, I almost always use mostly CSS for the layout, but there are still some areas that a table is a good idea (cart, reg form, etc). But that also means I need to manually install just about anything I add to their carts.
2. DO NOT give them a copy of X-Cart. They can download a demo to work with or work on your site. Which CSS forum are you talking about? And post count does not necessarily equal competence
Does he know X-Cart and Smarty? If not, he can do some serious damage to your site, so make a backup first!