@Stephanie1115 I would say something as neutral as possible, 'Thank you for letting me know you have decided to leave early. I'm sorry to hear that you did not enjoy your stay.'
Don't accept any responsibility for any of their alleged issues, 70F is pretty cool, so unless you were turning the temp back up during the night, it seems unrelated to the A/C. If a full bed was too small for them, that's their own fault. Don't spend energy trying to explain anything or asking them why if the trash smelled bad, they never said anything...all of these details are only important if they want a refund and you refuse to give them one, otherwise it's water under the bridge. They will probably leave you a bad review regardless, even though so far, they have not asked for a refund.