If your friend is leasing the mare, there should be something in the contract stating just who is supposed to be paying what when it comes to her care.
Your friend going on vacation doesn't mean she can shirk her responsibility to the horse when it comes to foot care or feeding if it's in her contract.
Horses aren't dirt bikes. Someone
needs to be responsible for the animal's care, regardless of whether or not the person leasing the horse is riding it.