first off welcome to the site
and can you answer a few questions ?
are her feet cold ALL the time, or only when hunting!
as if there cold all the time, you might want her to talk to her DR< might have blood flow/circulation issue's that are causing her the cold feet!

next if NOT cold all the time
can you explain when they get cold?
most cold feet when hunting.sitting on stand, come from sweaty feet issue's, and then when the heat expires from time, that moisture makes things COLD !
good socks are major role in keeping feet warm and dry, same with boots

but JUST adding extra thick socks or boots with heavy amounts of insulation, WON"T make feet stay warm, as if they sweat, or have poor blood circulation problems, all the insulation in the world won;'t help
nor will having heated socks/

SO< to the OP<> if you can describe when and where your wife is trying to keep feet warm, we might be able to offer better suggestions/recommendations!

and I have also tied many pairs of electrical heated socks, gloves, jackets and so on,and IMO< there all JUNK, and more gizzmo and gadget like, than real ways to keep warm,a s once battery's get low or die, your back to having cold and poorly designed products to keep you warm without power!
better things are out there that work without batteries!