Honestly? I wouldn't get involved, if the payments stop or aren't on time or the debtor in some other way reneges, then the go-between will start receiving the messages from the person who should have had the money. I can almost guarentee that whoever takes on the go-between role will end up with a worse relationships with both people they are trying to help.
If the debtor doesn't want to get involved in a conversation, they can just transfer the money from their account, if they don't know the account details, and don't want to email for them, they can mail a check, or get a solicitor to handle the transfer, or they could drive past the house and post a bundle of notes through the mailbox. Refusing to act as a go-between will not stop the debt being repaid, sounds like a ruse to avoid doing it and put the blame on someone else.
You didn't lend or borrow the money.