If you move forward - ESPECIALLY amongst friends, good friends, a contract is highly recommended. Nothing worse than a mis-understanding to destroy a friendship.
My contracts have always outlined the replacement value of the animal that's not in the original owner's care, should it die in the other person's care and the length of time needed to pay that restitution.
What happens if your large female were to die in your friend's care? What do you place as her replacement value? Who's going to be responsible for vet care if your animal gets sick in your buddy's care? All those things need to be spelled out in the contract if you decide to move forward.