If dinner's not going to be a surprise, just ask your mom what she can handle. There are lots of triggers out there and they're not always the same for everyone. For instance I can have dairy, including ice cream, but that's horrible for a lot of people. I can't have caffeine and some people can.
The worst offenders--those things that bother most IBSers on this board--are caffeine, carbonation, coffee, artifical sugars--especially sugar alcohols, apples and apple products, alcohol, raw vegetables, high fat, and greasy or fried foods.
You might try a stir-fry with chicken or fish and choose softer vegetables, like baby corns, onions, snap peas and water chestnuts, to put in it (rather than harder-to-digest broccoli and bell peppers) and serve over white rice. Egg drop soup shouldn't be too bad as a side. Dessert should be small and non-dairy.