Based on my looking, the tax is applied/not applied to an item not a customer, so there doesn't appear to be a way to waive sales tax on a purchase for a particular customer without going to Tools-> Options -> Tax and selecting "No Tax" as the default tax option, then ringing the customer out, and going through the same steps to turn taxes back on for the next customer. This would of course only work if you have all items set to use default tax setup, not their own tax settings.
A "Tax Exempt" button would be nice during checkout for you, a worthy enhancement request you should place through their support line.