What would completing the Co applicant line make different if he was not a cooaplicant? There is no obligation a person be a cotenant generally. He could simply be an occupant under her tenancy.
I dont see the tenant being at fault at all here. From what you’ve written you accepted her application with just her as tenant. His financial condition wasn’t relevant to you renting the apartment to her.
You refused to rent to her once you found out who she was going to have living with her. She didn’t ghost you.
Not only do you owe her the holding deposit, she might actually have a claim against you for her damages.

