The law is not always clear .... and there are ALOT of legal issues that could arise. And since it's YOUR property and you are agreeing to horse babysit/care, the burden of ANY damage to property (yours or someone else's) and/or injury to you, your family, neighbors, people driving by, her or her family would likely fall on YOU. You've heard the saying that posession is nine tenths of ownership? Well, the horse is on your property = it is in your posession. Hence the legal ramifacations do not favor YOU.
Case in point: if a child and her family are walking down the road, and she sees the pretty horsey and runs over to it and it bites or in any way hurts her, is it the supposed owner's fault or YOURS?? If you are going to say the owner's...don't bet on it.....YOU could be held liable even more so, as it is YOUR property, hence the safety of ANYONE and EVERYONE who walks, crawls, drives or floats onto your property (whether invited or not) is YOUR problem.
Even if you decide to not offer to horse sit or care for it....IT IS YOUR PROPERTY, IT IS YOUR PROBLEM
Imo, What you are considering is legal and financial suicide.
What I can't understand is if you want a horse to care for so bad, why not get one from a rescue?? Or a freebie that can only be used as a pasture pet?
Why cut your own throat by involving yourself in a highly legally risky arrangement that could end up as your worst nightmare?