Franz Josef has the limited choice of a small tourist village — a dozen or so cafes and restaurants strung along the main street, most of them workmanlike rather than memorable. The good news is that quality is consistent and prices are reasonable for New Zealand.
Snakebite Brewery does Asian-fusion food and decent craft beer in a busy, casual room — the busiest place in town most nights. Alice May is the long-standing pub-style restaurant with reliable burgers, lamb shanks, and West Coast fish. King Tiger for Thai. The Landing Bar at Franz Josef Glacier Hotel does standard New Zealand mains and is the closest thing to a proper sit-down restaurant.
For something quicker, Picnics Cafe on the main road has good coffee and pies. The petrol station also has the only late-night options if you arrive after the kitchens have closed.