http://www.airsoftcanada.com/showthread.php?t=122780
http://m.instructables.com/id/Airsoft-M72-LAW-launcher/
http://www.airsoftforum.com/board/to...aw-directions/
https://youtu.be/ZJ18JooBJW8
There have been home brewed and pre made airsoft ones around. Id say take the best of both worlds and build it, love to see it when its done!!
You could go pneumatic or (as i would go) fit 40mm shelss with a trap door for fast reloads. This will also allow the use of it as a bb shower or to launch nerf footballs or simaler.
The links should give some basic ideas and some more searching and brainstorming should provide some more ideas as well.
should be some way to maintain the original trigger (or at least keep it in the same place, perhaps put a small bracket that lets it slide or rock/tilt then connect that to a rod and pin to hit the valve. Perhaps even find 40mm launcher parts and use some of those. Would be really helpfull if he has a nice shop and the ability to cut/bend metal.