Residential leases tend to have provisions continuing the expired lease on a month to month basis AT THE SAME TERMS as expiring. You'll have to read the lease to see if that's what it says. If it says that, then you continue to be a guarantor for as long as your daughter and the bf occupy the rental.

You saying otherwise does not absolve you of the obligation.

Read the lease, what does it say about expiration? Quote it word for word. And what does it say about your obligations as co-signer?