One kind of random thought here, if your friend would do the vacinnations for you.
Some less honest people (not saying you, this is just hypothetical) would sue the friend if the horse had a horrible allergic reaction and died or needed expensive vet care, claiming that the friend did the shot incorrectly.
Would you want to put a friend in that position? (Unless you sign a waiver releasing them from liability)
Just posing a devil's advocate question, since our society is so "sue happy".