Who sent you 53914a3fac94780cecb74b0e77b76525c0ec5baf57656218b6e45e503766d446
It looks like that txid was sent 2 days after the loan was made (it was due in 3 days), and it was for the exact repayment amount. It looks like that transaction was from
this person, or at least it was claimed to be at the time.
You really should provide each borrower with a new/unique repayment address so it is very clear that it was them that sent the repayment.
If truly necessary, I would say that they could sign an address with it. But, I agree with you.
Will do so next time.
EDIT: Fraxinus admitted in PM to not being able to pay the loan (at least not fully), so it definitely could not have been him sending that transaction.
Also, a pretty big indicator could be that nickaizoku's address is the same as the address that sent the bitcoins.