Why did you not realize that “this guy is worth it” until he already has one foot out the door?
If his only reason for being dissatisfied with his current job was the money, why didn’t he just go to his manager and say “hello, I was looking at salary surveys and other information about the job market, and reviewing all the ways that I have made money for you guys over the past year, and I think I deserve such-and-such more than I’m getting right now”? Why is he finding staying in the familiar environment of your company less attractive than job-hunting in his spare time, possibly even burning vacation time for interviews, and then going out to work with a bunch of strangers?