Am I obligated to return the money my friend sent me as a trust, after a large portion of it was withdrawn from my account due to a fraud operation, knowing that the remaining amount is less than half of what he sent?

If money is stolen from a bank account where someone else's money was deposited without any transgression or negligence on the part of the account holder, then the account holder is not liable for the deposit because it is a trust (amanah). The remaining amount in the account is to be divided between the account holder and the depositor in proportion to each person's money. If a portion of the shared money is lost, the loss is considered to be on the partnership, and the remainder is divided between them according to each party's ownership percentage.