Friends opening their own restaurant might still need some specialised cookware, chef's knives or other utensils for the kitchen. Another idea would be a wall hanging for the entry foyer or silk flower arrangements to put in the bathrooms.
After the time they've invested to get the restaurant open, these friends could probably use some pampering. A bottle of wine, a gift token to a spa, a fruit basket or treating them to dinner would be appropriate.
If you're sure of the cooking supplies your friends need or that your decorating tastes will match the establishment's decor, buy them a restaurant-related present. If you don't know what restaurant items your friends could use, stick with a gift like wine or a fruit arrangement.