I think it all depends on how long she will be there for. For a month, you can find [very] basic accommodation for 150-180RO most likely. Daily, budget accommodation will probably be in the realm of 12-15RO. It depends also on what you mean by "affordable." A couple of the budget options are in the Muttrah area. Hotel Nasseem is the only one I have experience with, and is one of the main ones.
Daily living items are fairly cheap when bought from the small supermarkets/"cold stores." Water and soft drinks cost about 100 baisa. This figure will go up depending on where you are. A plate of biryani costs around 1 rial. A single shwarma sandwich is ~300baisa. Larger sandwiches are between 700baisa and 1 rial or so.