Vegetables and fruits make for great treats for some fish, but I would not use it as a staple part of their diet, as it will not have all the needed nutrients that fish food supplies. Rather, make it a once or twice a week thing to bring some variety into the diet. Of course, take what kind of fish you have into mind when deciding what to feed them. Carnivores will laugh at you if you put some vegetables in their tanks.
Personally, I use slices of cucumbers, zucchini, and yellow squash for my pleco. I have a clip on a suction cup (which you can probably find at one of your LFS's for $2 or so) which I put the slice of vegetable in, usually before night time since my pleco likes to hide during the day. The next morning, everything but the skin is usually gone.
Other things that can be fed include peas (with the skins removed), broccoli (just the stalk part of it), lettuce (sliced up), and some fruits. I know I heard of someone giving oranges to their goldfish, though I personally have never tried feeding fruit to any of my fish. Just remember to thaw out anything frozen before putting it in the tank, and consider boiling it and then letting it cool to make it soft for your fish (also not that some people insist you should boil every time, and others who say that boiling is completely unnecessary). Really, what you needis a lot of experimentation to see what your fish prefer.
Anyways, good luck to your mother with the veggies, and kudos to her for looking into treats for her fish.
