[heRS]

Gary, I am chiming in as a customer service professional here...
Customer ordered from you for a project he needs both pieces to finish. You have been unable to satisfy him by providing both pieces and an unreasonable amount of time has gone by.
You should have UPS pick it up and refund his money, or supply him with the part that should have been delivered in a timely manner.
IF you have problems with YOUR supplier, you should NOT hold your patient and frustrated customer up any longer. He should not be in this position, and you, as a merchant wishing to come across as professional, should offer to take the product back. I already would not order from you because you come across in an inflammatory manner and your customer is being quite reasonable in his request.
You also state that for the past year you have been offering another upset customer the option to return a product and yet you are digging your heels in on the OP. Where's the line? You return for one but not another? Consistency would be appropriate.
I recently was held up for over 3 months on an order for a pair of working boots for my husband. I will never EVER buy from Chippewa boots again and I suggest y'all follow suit because they come across like a bunch of entitled ingrates. I requested my money back and got it but not without a fight. Instead I am spending $375. more and having custom boots made for him. Someone who is giving me GOOD customer service is getting my business now.
Any road, were this case on my desk I would be refunding the customers money and apologizing for the entire situation profusely. You will make more money in the long run by coming across as a hero instead of a zero. One dissatisfied customer can draw a LOT of attention, as you are finding out here. Just give the poor guy his money back and let him get on with his project.