Echo everything Jhumfy said! As someone who works part-time in the equine industry, I feel your pain - it can be ridiculously unforgiving and difficult work, both mentally and physically. Horsey women (I generalise here) are a force to be reckoned with, often have absolutely no manners/social skills (and no willingness to even TRY and be civil) and can basically just be a bit of a pain in the arse. They can sometimes be won over with vigorous agreement to everything they say ("yes, strapping your horses head down will definitely improve his jumping ability, I can't believe I used to think any differently!!!" *nods encouragingly whilst trying to hide look of horror*) and numerous cups of tea, but honestly, if you're just there for a week, I would just try and brush it off as much as you can and struggle through. It does seem like hell at the minute but realistically, it is only 5 days of your life, and unless they start physically/verbally/sexually abusing you (if they do, you need to leave immediately and report them to the BHS and local police service), I would try and finish. As you say, equine husbandry is one of the main things that the vet schools want to see, and it *may* put your application at risk if you don't have it on there.
As for alternatives, you could have a Google and see if there are any other riding schools that would be willing to take you. It is peak season and most are usually happy to have an extra pair of (free!) hands. If you are okay to make your location public on here, you could also ask if there are any members that know of any near you that take people on. There is also a very busy UK equestrian forum called Horse & Hound Forum that you could make a post on (if you do, make sure you don't mention the name of the place you are currently working at - anyone could be lurking!!).
I really do empathise with you, but spending time working in a difficult environment will absolutely work to your advantage one day - you will walk out of the placement covered in horse hair/snot/fecal matter, and with tirades along the lines of "WHO LET THAT PONY/DOG/SMALL CHILD IN THERE" ringing in your ears, but you will have experience working with horsey women, and that is a valuable thing.
(Having said all of that, your safety is far more important than that of any application, irrelevant of whether it's to vet school or not, so if you really feel that you are, at any point, in a dangerous situation, get. out.)