Join Date: Sep 2009
Location: Tampa Bay area, FL
First of all, I would get a contract. Even if she is a family friend, business is business, and a real friend won't take offense to wanting to cover everyone's backs in case of an accident or misshappen.
The amount of responsiblity depends on the kind of lease. If it is a half lease, you and her will be responsible for the horse 50/50. If it is a full lease, you will be responsible for the general well being of the horse, with all major decisions (like being put down, major surgery) ultimately up to the owner.
At my barn, a full lease is 250 dollars a month, with the leasee paying half of whatever vet bills the horse incurs (usually just routine checks and vaccinations) and unless the horse is shod, the farrier bill is paid for by the BO.
You should know the same things you would need to know to actually own a horse, the basics of first aid, how to groom, tack up, and ride a horse.