If I were in your position I would be having that hoof x-rayed. You don't know that it didn't nick the bone or cause infection within it. You mare should have also had a tetanus booster at the same time.
I as a barn owner would never tell a boarder the horse will be fine after an over 1 inch piece of metal came out of the hoof. That's a pretty deep puncture and just shoving antibiotics in it just doesnt cut the cake. A puncture in the foot must be soaked daily and wrapped to prevent any foreign particles to enter. She may seem fine right now but I bet you there is a pus pocket fuming in that hole. I sure hope I am wrong But I would soak that foot in as hot of water and epsom salt as you can stand. I would even add a table spoon of Lysol ( BROWN BOTTLE YELLOW LABEL) to the water.
This disinfects the wound. Heat doesnt always indicate trouble.