If the father had done a better job parenting instead of permissively coddling his children or allowing others to parent them, he would never have had to write this email.
Parenting is coaching. It's a process of constantly weening children from dependency after dependency. It can be tough when they cry at every rule you put in place and at every missed chocolate or toy or tv show. But you have to be firm, stay true to your vision, and view yourself as saving your children from "the tyranny of their own desires".
If you want it done right, you can't do it from a distance. You can't oursource the effort by sending them off to boarding school and hope someone else will do your job for you.
It's not easy. I have 3 of my own. Perhaps the hardest thing to do is to realize that the way your kids turn out is largely due to how effectively you and your partner parent. Yes they make their own choices. But they should make those choices within a framework of confidence, intelligence and independence that you as a parent have cultivated.
That's a lot of responsibility. But you have to take it on if you want to increase the chances of your children having good lives.
Waiting until they're 40 to write them an email expressing your disappointment is far too little way too late.