I strongly second the slowfeeder haynet. Keeps her busy.
I've seen this kind of cribbing with horses who didn't have food for long hours / been stalled without sufficient roughage. So don't be mad at the mare, rather at the people who did this to her.
The slowfeeder will also help the colt not picking up the " habit"
Slow Feeding Horses on Paddock Paradise Tracks - Paddock Paradise Wiki has a variety of ideas for slowfeeders to show to your grandparents.