Sorry that happened to your friend. Was it proven that the driver was stoned? If so I'm guessing they have an accurate way to measure it. Either way this is always going to be a problem. Look at the number of people killed by drunk drivers. The only solution is harsh prison sentences imo. Some people, willing to take the risk of DWI, disregard the danger they pose to others and if they can't understand that they should be severely punished. DWI shouldn't just lose you your license and get you probation. The punishment should be much more severe.
As for your question on the legal limit - there shouldn't be one. If you get in the car you should not have had any marijuana in the last 24 hours.