Tough one. I would reply with something along these lines:
"Dear Misty, I'm so sorry to hear about your mom's health. It's a good thing she went to the doctor and caught the issue early on. It sounds like she has a good doctor and is in the best hands. Thank you for being straight forward and informing us of the situation regarding not being able to sustain the monthly rent for this unit. There are charity organizations that you may be able to apply to in order to get assistance on rent, and we would be happy to accept rent from such an organization (insert charity names and contact info if you know of any, in my area there are several grant programs that help single women specifically). Or, if you need a few weeks to find a less expensive place, we will let you out of the lease agreement with 30 days notice. Unfortunately we have our own expenses such as bank payments, taxes, insurance and maintenance that require us to rent the apartment at the set price. Wishing the best for you and your mother during this difficult time. Our prayers are with you that the surgery goes well and she recovers quickly."
It's especially sad when evictions are due to health issues. 100% of the evictions I've had to execute had some type of health issue component, either real or made up. I'd wager that a high percentage of evictions are due to health issues. Everyone gets sick or injured eventually, and it's expensive to deal with. As a landlord you can't act as a charity, you have your own financial obligations to your lender, your investors, your reserve funds, your other tenants, your family, your goals for growing the business, your own health issues, etc.
Also as an aside, she should have told you from the beginning that her mom was subsidizing her rent, and you should have screened her to make sure her individual income was 2-3 times rent.