I recently became friendly with the owner of a Thai restaurant.  She
was forced to close while the landlord made badly needed repairs to
the building.
 Unfortunately, the project has run considerably over the allotted
time, and my new friend has been without the income the restaurant has
produced for some time now.
 She's a warm, generous person and a good client, too.
 I am looking forward to the reopening - when Thai people open a new
business or reopen an existing one, what is a traditional gift given
to the business owner?

A couple of things to keep in mind when giving gifts in the Thai tradition:

Gift giving in Thailand has Westernized and is less formal than in
other Asian countries
The number 3 is considered a lucky number
It is acceptable to wrap gifts in brightly colored wraps and ribbons,
though don't be offended when receiver does not open the gift in front
of you as this is tradition as well.


http://www.lovingchocolate.com/html/gifts.html#thailand

Puget Sound International Gift Givers Guide 
http://www.cityofseattle.net/tda/giftguide.htm

Thailand 2Know
http://www.asiasource.org/business/2know/thailand.cfm

Oh. And I should mention that it's *not* the number 3 that's lucky for
Thai people. It's the number 9.

My friend's wife is Thai and I've traveled to Thailand several times. 

Here is a site that I have ordered from before from within the U.S:
Our Little Thai Shop: http://www.shopshot.com/thaishop/

Generally speaking, when Thai people give a gift for a special occasion they
usually give a small wreath of fresh flowers. Other gifts that are
popular would be Buddha images or Thai Buddhist related gifts such as
amulets or pictures of Thai monks. Jewelry is also popular to give.