If you have switched to Monzo from another bank and a payment was sent to your old bank details, the money should be redirected as part of The Current Account Switch Service.
Your old bank should inform anybody who sends money to your old account of your new details, as well as perform a redirection of the money to your new account, however in practice most just redirect the money or the sending bank/company ignores the new details.
However, this redirection is not instant, and some banks even perform it manually! 
Providing you have used CASS, I would give it a working day or two before chasing it as it entirely depends on how fast your old bank is at redirecting payments. In the mean time, make sure to give your friend your new bank details. 