If a question regarding non-interest bearing notes mentions that the customer is somewhat of a credit risk and typically borrows funds at a rate of...

If a question regarding non-interest bearing notes mentions that the customer is somewhat of a credit risk and typically borrows funds at a rate of 15%. The vendor is much more creditworthy and has various lines of credit at 8%, for example. How do you know what rate to use when calculating the fair value of the note. In other words how do you know what the implicit rate is and the market rate is? Thanks