The casing needs to be installed into solid bedrock. The reason why he is getting muddy water is probably because the casing is installed in weathered rock and the water above the rock is perculating into the hole through the voids. Or they might not have grouted the hole properly.The seal should be installed below the bottom of the casing and into solid rock. That way the water just hits the seal and cant get by it.
I usualy don't have to push the seal any farther than 10 to 15 feet beyond the casing. Regardless, a camera inpsection is necessary to determine where the seal should be placed. The video tells all. On some wells most of the water that the well is producing is coming in through that opening at the end of the casing and installing the seal cuts off a lot of the production. Knowing what the well originaly produced for water, and looking at how much water is coming in through that opening, you can get an idea as to how much the well will be producing once you are done. Sometimes you come across wells where 80 percent of the water is coming through that opening and instead of cutting off all the water the customer decides to treat the water rather having almost no water at all.