Has the tenant been making payments towards (or accumulating credit from rent) the ultimate purchase, or is the option literally just a right of first refusal? Not seeing the agreement makes it difficult to guess how to answer your question. Does the option say it terminates with the lease? I would expect you to have a reciprocal right to terminate the lease, absent a provision on point. However, if the tenant has credit towards the purchaser price, or has been making payments towards it, you may have a more difficult time terminating. Bring the agreement to a local attorney for more thought on this.
I may be guessing or not licensed in your state. No atty/client relationship exists.
Dear Rochester Landlord:
You may have crafted a lease where you waived the statutory right of a landlord to terminate a month to month tenancy without cause. In an ordinary month to month lease, there is no reason for the landlord to reserve the right to terminate the tenancy based upon a default since the statute allows for termination on proper notice for no cause at all.
If there is an ambiguity in the lease the ambiguity is construed against the party drafting the lease, likely the landlord.
Because you have a written agreement, you need an attorney to review your lease and answer your questions.
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.