If the keypad wasn't working strange they made no attempt to contact you and were able to get in the next day without a problem, @Linda3182
If you are confident there are no issues with the key pad, I would say to the guest you are puzzled by their complaint as they didn't contact you at any point that evening to say they were locked out . And that when they used the lockbox the following day it was working .
this being the case you wouldn't be able to consider their request for a refund .