If it's during shop hours, I'd do away with the food and make it a free event. The shop owner can have bottled water and coffee (a Keurig with an assortment of K-cups would be great in this instance) and perhaps pre-packaged snacks (pretzels or crackers, granola bars, cookies) available for purchase at a modest price.
You want to encourage people to come into the shop, see what's available, and hopefully purchase something and spread the word.
Now, if people start wanting to use the shop's sewing machines or ironing boards, they should be charged a set fee for doing so.
Hope this helps, and best of luck to your friend on her new venture!