Now, in order for the bank to be able to be able to pay its debt to your friend one day, bank has assets. I.e. someone has borrowed money from the bank (e.g. mortgage), and those assets can be used/solde to pay your friend if your friend really wants the bank to settle its debts to him.
It kind of is turtles all the way down. Someone needs to be in debt for there to exist any money. Money is a really weird kind of bearer note. It just means that if you have money, you are owed some valuable goods by other people. And you are pretty free to choose who pays that debt to you from them who is willing to take that bearer note from you as a payment.
Note that debt is also how money is born. Technically everything a bank does when it adds for whatever reason money to your bank account is that it increases your balance in the database and boom, we have new money. Of course, usually banks are not stupid, and in order for them to increase their debt to you, they want something from you in exchange. Typically a promise to pay back a bit more some later day.
(As a disclaimer, money is really tricky to think through. So I give no guarantees my thinking is correct, but so far thinking money as a debt has been most useful way for me to understand it.)