Reuvan rented a dira from Shimon. He was given by the previous tenant 2 keys for the dira. Reuvan lost both of the keys and asked from Shimon a copy of a key in order to make more copies for himself. He made more copies and returned the key to Shimon. A long time went by and eventually Reuvan lost ALL of the keys he had made. He again called Shimon to ask for a copy of the key he said would have to look for it. A day or two later the Bal Dira, Shimon, calls back saying he also lost his *only* key. Now they have to change the lock on the door and make a new key. Who is responsible to pay for this?
(And if Reuvan *is* the one responsible to pay for the lock, when he leaves the dira may be take the lock he paid for and put back the old lock that both Reuvan and Shimon lost the key for?)
Reuven is liable to pay for the keys, but not for the lock.
Unless he makes it clear in advance, it is reasonable to expect the owner of the house to have an extra key(s), and therefore losing the keys is not considered an action of breaking the lock, which would obligate Reuven for the payment of a new one.
Installing a new lock is a major job, which requires a tradesman, and therefore the payment for it falls on the landlord, Shimon.