Are we obligated to return the money to someone who paid to replace one of the tenants and then withdrew, or is he bound by the monthly rent?

The exact nature of the agreement with the friend is unclear. However, if an agreement was reached to rent the apartment for a specific period at a defined rent, and there was an offer and acceptance, then the lease is binding, and he is obligated to pay its rent. If the contract was without a specified duration, then he is only obligated to pay the rent for the first month. But if what occurred was merely a promise or negotiation without the formation of a contract, then he is not obligated to pay any rent, and he has the right to demand the return of the amount he paid.