raised beds

What do you mean.. "IF".. you make them for her is there any way you could attach one to the other and make a 18" high raised bed for her? With regard to what you can grow in one..the skys the limit, i've grown allsorts in mine, i think there fantastic you don't brake your back planting them up.

Yes

Most if not all fruit and/or veg.

Good isn't it?

As long as they're not on concrete 9" will be plenty deep enough. If there's soil underneath, any deep rooted plants will be really happy.

Due to living in a clay pit (or seems like it anyway) pretty much all my growing is done in raised beds. And mine are probably only about 9" high, give or take an inch.
I dug the soil over first, then filled them with whatever growing matter I could find.
Since then I have grown just about any veg you can think of in there.
Had a carrot close to a foot long this year, so the depth isnt as much of an issue as you would think, as long as you turn the soil over annually or you can get a solid base forming under the raised bed matter.

Ginger is correct - raised beds are what they mean 'raised beds' not only that if they are deeper you can grown deep rooted crops which carrots for instance need!

Mine are made out of old 5 inch boards, wish I had scaffolding planks ! They are three or four boards high. The more rain you get, the better they work if raised higher I think - the improved drainage makes more of a difference.

The main advantage is that they warm better and drain better, which helps the plants grow, and the weeds don't spread into them the same so you don't have to weed nearly so much.
Also the ground between them is a permanent path, which helps keep things tidy and weed free.
Last but not least, if you mound the soil then you can get more plants into a given horizontal area, at least for leaf crops.
I always put the uprights inside the beds, to make things easier when wheeling a wheelbarrow around, and because I remove all the soil when taking out tatties, I leave the end panels fastened by screws so that I can remove them more easily. That way I can get absolutely all the tatties and don't leave any in the soil to screw up next year's planting. If your ones are shallower, it won't matter so much.
You can make them with protruding uprights that act as anchor stakes, or you can just fasten the edges together at the ends - I try to have my uprights adjoining each other so that just two screws will hold the ends on. Don't make them too wide to reach across, and plan ahead for putting up netting hoops etc.
Hope that helps you decide, if the missus hasn't already !