I think I have tried ALL edging materials possible. And now finally I have decided on the easiest and cheapest: a trenched edge. Just a little trench made all the way around the bed with a half-moon edger, like this:

You push the edger down in the ground (I stand on it) and wedge out a triangle of sod, making a little trench. Easy peasy. If you are careful, you can make one long worm of sod that you can lift out in one piece (like making one long peel off an apple). Then you can touch up the trench as needed with a hoe (a triangle hoe works best) if grass/weeds/mulch falls into your trench. I usually do a good touch-up around all my beds once in the spring.

Cheap. Easy. No trimming required because it's easy to mow along the edge. Quite unlike trimming along assorted rocks or landscape timbers.

And, BTW, I think I'd try to convince your mother to replace the white stones with mulch. Especially if you have mulch in the bed next to the house. (That is, if it's necessary to keep that bed in the middle of the yard.)

Here's a picture of a bed with a trenched edge. I used to think this would be too hard for me to do (I am a 60+ year old woman), but I have since decided that it may cost a little sweat in the beginning, but over the long haul it saves a LOT of time. And looks great.

Since you used to have landscape timbers there, you already have a great start on a trench. Probably all you'd need to do is touch it up to make the edge even. Lay out a hose where you want the edge to be, then follow it with your edger.