First off, this isnt your horse, so you really need to be communicating with the owner about finding a vet or how to care for the horse.
A vet call is free. You could even gasp send email pictures to your vet. Its pretty simple to do. I will always call my vet, if I am not sure whats going on, even if its not a life threatening issue. They will usually talk me through it and ask me questions so they can better understand whats going on, if no one can be there, or the office is open for emergencies only.
Its not irresponsible to treat a horse on your own, but it is very irresponsible to not even try to call or email the vet. In some situations you could be doing a horse more harm then good, by doing what you think is the right treatment in your mind.