Well the newer part of Girona, on the west side of the river, is very flat, easy to walk, with wide streets, like any normal town/city. On the east side of the river is the old town, which is built on a sloping hill, it can get a little inclined in a few areas like this. On the way towards the Cathedral, however, it does get quite a bit more inclined as you can see here and here. There are usually steps for the more steeper parts, which will make it easier to climb up. I'm not sure about your husband's ability so I can't say a definite yes or no, but I can say that the old town area is definitely not flat.
I don't think a local guide is necessary, with a little research you can tour the city quite easily on your own, even if one does not speak Spanish.
As for a restaurant in Girona, I recommend Casa Marieta, in the Plaça de la Independencia. It's a venerable establishment with more than 100 years of history. For a traditional Catalan treat, try the Canelones de ceps, which are pasta tubes stuffed with ceps, which are local mushrooms from the area, and baked with a béchamel sauce on top. You can read more about the history of the place here: https://en.wikipedia.org/wiki/Casa_Marieta