This would be more clean-cut from a legal perspective if he had waited for them to *attempt* to break in (break a window, start prying a lock, etc) before shooting. The window will be a loss anyway whether it's busted with a hammer or a bullet. Firing through a glass window at someone who is standing close to your house is overreacting. I live on a corner and kids walk through my yard all the time. Sometimes they stop and stand around, but I'm not about to start emptying a magazine on them.
Additionally, when shooting from inside a structure at people who are on the outside, a hit on the first shot is not absolutely necessary. Normally I'm opposed to warning shots, but in this case, you have an incredible amount of cover, and they have none - they also have a lot of room to run. One or two shots into the wall next to the window would probably motivate them out of the area at a high rate of speed, and the rounds should disintegrate if you're using good HPs. You can always say you missed.