I've not had crash bars on mine, but I've had some bungs.
My bike went down in the ice a few months ago and I had R&G engine crash bungs fitted, they saved it receiving damage at all. It's been down once since, and that finished off the same bung. If it'd not already been worn, my exhaust and engine casing might have remained a little more scratch free, but both sorted, and neither damaged to a level that mattered (unlike if I hadn't had them). My previous FZS600 had the same ones fitted, and they saved the engine when someone pull out in front of me. The casing was still broken, but by hitting part of the car, and I was quick enough turning the engine off to be OK. Faring smashed by the car though.
I've now got a genuine Yamaha one on one side, and it's not as sturdy, the bolts are thinner and it doesn't stick out as much - so it'd definitely not work for more than 1 slide. The 2nd slide on the R&G bung resulted in the large bolt running through the frame to bend, but without bending the frame. Very clever!
I say - if you're only worried about her dropping it at lights, bungs will be fine. I'd be as worried about her picking it up, it's not an especially light bike.