Hi, I'm Dr. Bonk. I'm sorry to hear about Marshmallows troubles. The two biggest concerns with cuts on the hoof are keeping it clean and keeping it from gaping apart when she walks so that it will heal.
First flush it with a dilute iodine solution or peroxide to disinfect. Then apply an antibiotic ointment. Neosporin works fine. Then apply a non-stick pad and wrap the whole hoof with vet wrap or similar bandage. Make sure the bandage is not too tight but that it is thick enough to immobilize the fetlock joint so that the skin won't gap when she walks.
Keep her in a dry, clean area and change the bandage every day. Clean and flush the wound daily with dilute iodine. Don't use peroxide past the first day. If the wound develops a thick, foul smelling discharge, becomes swollen or extremely sore, you should start her on penicillin at a dose of 7 ml per 100lb per day.
You will need to keep wrapping the foot until the cut starts to fill in with the spongy pink granulation tissue.
If the wound isn't showing improvement or gets worse after 2-3 days, she should see a vet.
I hope this helps. Please contact me with any further questions you may have. Thank you and good luck!