I made lemonade out of lemons bc it gave me the excuse to constantly groom my horse's manes and tails. Though I really like Showsheen, and I've tried Pink, my personal favorite to detangle is plain, old, corn oil. If you coat the burrs with it they literally slip right off.
It's like this--I do NOT use any herbicide to kill burrs. I pull them, mow them, bury them and burn them. I have 1/2 the burr problem this year, 2011, that I had in 2010, and it will be better in 2012. I will be cutting the ones left and they'll be burnt in the middle of my north pasture where my burn pile lives, so it's doubtful that those will reproduce.
Personally, unless you want to help de-weed the place or help the owner groom the horse's manes and tails, I think the horses look otherwise healthy and you'll just pXss off the owner by getting involved. JMHO!
BTW, did you know that the leaves of the cockleburr plant soothes skin irritations and burns? (Thanks to Reuben, my little Amish farrier!!)