You might like Sweetie Pies in the old Hatt building by the river a few blocks away . . . some basic lunch fare, fun atmosphere, plus, they have irresistible desserts and coffee drinks:-) Also there is the General Store which also serves less expensive lunch fare. If you're hankering for cheap eats, try the middle east place called Small World - very close to Betty's Boutique.
Double Rainbow ice cream is right across the street from Betty's in the Downtown Mall area.
Is that Betty's Girl Boutique 1239 1st St, Napa, CA ? If so lunch at Oxbow Market 660 1st across Soscol. Try Pica Pica. If it is a nice day eat out back on the porch.
Several other interesting choices also in the building.