I happen to have a degree in Horticulture, and I have to say, I kind of think this won't work, but...
If you were to graft the stalk back to the base, which is actually pretty easy, just make a V shaped cut, facing down on the stalk and a matching one on the base (does that make sense?), then, wrap it with a strip of rubber, specifically, one of those thick rubber bands, cut into a strip.
It might work.
I've seen some really weird things with sunflowers in experiments. I once cut one at the base, flushed the soil with red dye, watched the dye ooze out of the cut, and put it back together again and the dye made it up to the flower petals.
Don't know if it'd keep the plant alive long term, but it couldn't hurt.
Oh, of course you'd need to stake it, like LQ mentioned.
Really hope it works or you find a solution.
You're a good parent to want to shield your daughter from something like this at her age.
Can he find a local nursery where they could put one in a pot and he could replant it? I gave him a link to a list of at least 2 dozen nurseries in his area.
Do you think a nursery in MN has them in the summertime?
I have seen them in Kansas in the fields, TX and near Carmel in CA.