I am not sure if there are currently grand marshals in parades. Back in 2003, people were randomly picked. I know of a honeymooning couple wearing Minnie and Mickey bride and groom hats, and pins who was in Hollywood Studios and looking at ride closures, when a friendly CM approached them, asking them where they were from, if they were having a good time, if it was their first visit, did they get married there, and if they wanted to be grand marshals in the Stars and Cars parade. (Parade now retired.) They had a magical day, got to sign a huge (sorcerer mickey style) magic book, received a bunch of unlimited one day use paper FPs, got to see front row Beauty and the Beast Show, and this wonderful CM used their camera to take pictures during the parade route. (And of course memories to last a lifetime.)
There was no way to ask, or volunteer or nominate someone, it was random luck, right time right place situation. out of all the people in the park, only one 'family' was chosen.
(check out the link in my signature for 2003...and you will see how I know a bit about this...)
I do not think the procedure has changed, if there is even a grand marshal in any parade...?
It sounds like you might want to consider doing something for this person...even something as small as noting a special occasion in an ADR...I would even call it a "magical triumph"... There are some totally lovely items in the Disney Florist's website. One gift, "Magical Triumph" would be a good sweet choice! I think it would be a wonderful gesture to surprise her yourself!!