This sounds like a Monzo user used the “Pay anyone with a link” option which asks the receiving person to enter the account details of where the money should be sent.
Thus it’s the originator of the payment who has the Monzo account, not the person who received it - hence the Co-Op details. So it’s the originator of the payment who needs to contact Monzo and tell them that their friend entered the wrong account details so the money is missing. Monzo will need to contact the Co-Op Bank and see what can be done.
Sadly, this is exactly the sort of problem that this payment mechanism is supposed to avoid on the assumption that people will get their own details correct more easily than the details of other people’s accounts.
@Patg I don’t think there’s anything you can do about this at all I’m afraid since you are, effectively, not involved in the transaction at all.