That seems almost counter productive on a few levels.
1) While it may deter this thief it will not likely deter others.
2) Said thief (from above) might well return later and trash the outside of the vehicle in retaliation.
3) The owner still has to replace the window.
4) The flash bang may have left residue in the cab which now has to be cleaned up.
5) Depending on local laws it might be illegal.*
6) If a thief were to be injured he would likely be able to sue the owner.
While I can well understand the desire to get even with these pieces of dog crap that do this stuff I don't think this is really the most effective thing to do - however satisfying it may be upon initial glance.
*In Colorado (which has a really strong home affirmative defense law) it is illegal to booby trap your car, business, or home with anything that can seriously injure or kill someone.