Casual, low-priced, great food near Sheraton?
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
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).