glock google glock leg syndrome LOL

Don't aim for anything but center mass

Your friend would be well-advised to do some reading, perhaps Massad Ayoob's In the Gravest Extreme, Bill Jordan's No Second Place Winner or almost anything by Jeff Cooper. All of these will be somewhat out of date regarding hardware but they remain outstanding for the sake of self-defense mindset. Hopefully after such reading your friend will withdraw the question and begin to consider all the other questions that might save his life from aggressors and his property and freedom from attorneys.

If you have adequate Home Owners Insurance to pay the pending lawsuit just about any will do. For a better chance of success to disable the victim, a shotgun with any size bird shot will do. If you only want to hit the leg just a little then use a 410. If more destruction is desired then a 12 gauge will put more pellets in a wider pattern. A 12 gauge is better for those that close their eyes when the trigger is pulled.
For those without insurance, a healthy bank account will help pay for the victims medical bills and disability for the rest of their life.