You need to make a formal complaint to Santander.
Think of it like the postal service - if a package is lost, it’s the sender who needs to make the claim with the postal service. In this case, the sender [you] needs to make the complain with the ‘postal service’ you used [Santander]. So I’m not surprised the complaint to Monzo got nowhere.
Even if it ends up being Monzo’s fault and that they’ve got the money but not deposited it properly, it needs to be Santander putting the pressure on them on your behalf to sort it.
My advice of a week ago still stands: make a formal complaint to Santander. Make them trace and retrieve your money for you.
Santander will not ‘pull’ any funds from your friend’s account - they will try and find the money you sent. They will have to find that specifically allocated money in order to resolve your complaint, they can’t simply ‘retrieve’ the sum you sent from your friend regardless of where it came from.