"This is actually the potential hire's chance to pay me a compliment, give a small amount of feedback, and keep the door open for the future in case their other "better" option falls through."
That's quite narcissistic and self-absorbed. You're really not that important and to take a letter like this and try to paint it as a "blessing in disguise" is arrogant and foolish.
He doesn't want to work in a Windows shop. He wanted to be able to work with particular technologies. He's a new grad and idealistic. He has preferences and opinions and had the insight to skip working in an environment he believed he wouldn't do well in. He's not a bad hire, just a bad hire for your organization. Get over it.