Casual, low-priced, great food near Sheraton?
Stanley's for breakfast or lunch, right on the corner of Jackson Square; EAT on Dumaine (I haven't been, but friends like it); WINO in the Warehouse district if she wants wine and snacks; if she's willing to explore a bit, I'd highly recommend Mimi's in the Marigny - the best value tapas I've had anyplace outside of Spain (in terms of taste to cost ratio - extremely delicious:pretty durn reasonable).
assuming you mean the Sheraton on Canal St.
in the Fr. Qtr.
Muriel's, Mr. B's and Bacco do lunch specials and would be comfortable for a solo diner.
for cheap eats, try Mena's Palace, Coop's Place, Country Flame, Felipe's, Fiorella's, Jazz Taco, Central Grocery, Italian Barrel, Johnny's Po-Boys
in the CBD
Rambla, The Store, Luke, second floor food court at 201 St. Charles Ave., Mike Serio's Deli