Who wouldn't leave for more money? What did you expect? Did you give him a counter-offer? Now that you've invested time into him, he should be worth more to you

Who wouldn't leave for more money? What did you expect? Did you give him a counter-offer? Now that you've invested time into him, he should be worth more to you as an employee. Now you get to suck it up and train someone else, and spend hours interviewing replacements. Probably would have been cheaper to match the other offer.