I would add:

Betadine
Corona (works wonders on hock sores/pressure sores)
Sand-Clear or similar psyllium product if the soil is sandy and the boarding stable does not provide
A few plastic spray bottles
An extra fly mask so you can rotate for washing or if the horse destroys one, although your friend might want to wait until she decides what brand fits the best
I've read you should always have a knife around to cut a halter or lead rope in case of emergency
Coolers, rainsheets, and blankets will be on sale now during summer
COOKIES!!!