Whoa! Ouch. That underpass is way too skinny for its own good. Could happen to anyone.
I hope she has one of the high tech casts that you can mold to your hand with heat and take it off for washing. I had one for my broken wrist last year and it was a vast improvement over fiberglass ones (though still not a joy to wear). Glad to hear it's not her dominant hand because that would be a PITA. Fortunately small bones heal fast; getting the muscles loosened up and flexible again is what takes forever. I've broken my wrist and a thumb and all are healed up and fine now; no lingering effects. Sorry a nice ride got spoiled; hopefully she will be riding again when the warm weather is here to stay.
Hopefully she heals up quickly. But tell her that I am incredibly impressed with her stoicism.