A co-signer to a contract promises to perform the duties called for in the contract. If the other signer elects to breach his duties under the contract, the co-signer is expected to perform. That is the entire reason for a co-signer. If the co-signer could just walk away when the signer breached, there would be no purpose to having a co-signer.
Your question says "the lease expired" without explanation. If the lease expired, the parties should not have any further obligation to perform.
I am not offering legal advice regarding your question, only general information regarding the law. You are not my client nor am I your attorney unless we sign a retainer agreement.
Dear how can I stop a car lease agreement?
Perhaps I might help. You did not offer the relationship you had with the vehicle lessee which induced you in the first instance to guarantee the lease. Your relationship with the lessee is an important fact for an attorney to consider.
Usually, the monthly lease is paid to a finance company and the dealer is not in the picture after the lease is negotiated and the initial payments were made other than service of the vehicle. If the dealer controls the lease and the dealer is the lessor on the lease you need to compel the dealer to comply with its obligation in the lease namely to take back the vehicle on the conclusion of the lease period. If the dealer is in breach of this obligation, that might suspend your guarantee. You might also seek to compel the lessee to correct his breach of the lease. To do that you will need to initiate a court proceeding and by court process make the lessee pay the lease amount as long as he keeps the vehicle, return the vehicle because the lease concluded and pay back to you the payments you made on the "expired" lease to protect your "credit."
You may need to hire an attorney to extricate you from this situation.
The answer provided to you is in the nature of general information. The general proposition being that you should try to avoid a bad outcome if you can.