What should he do with the money he has accumulated as a result of his work in an interest-based bank? And is he sinful because of this work?

You must strive to identify the owners of the amounts you received and did not deposit, especially since you threw away the receipts and no longer know their owners. This should be done by contacting the customers and verifying their identities. As for your work in the usurious bank, it is forbidden by Sharia (definition), and you did well to resign from it. You must not return to such work, for the paths of what is lawful are many.