I fought the exact same issue with my Jeep JK Unlimited. Scary as hell and no fun while at highway speeds! Replaced all of the front end suspension and steering components one by one until I had it cured. The biggest problem is, once your vehicle has had the DW a few times it accelerates the front end component wear/tear.
After literally days of research and troubleshooting, the following was the cure I found;
First things first, tell your buddy to replace the upper and lower ball joints. I ended up installing rebuildable heavy-duty ones that were expensive but well worth the $. At the same time, and most importantly, check the upper and lower bolt holes on the mounting bracket for the track bar. I would be willing to bet that the bolts holes are slightly ovaled (is that a word?) from the DW. Either that or there may be enough play between the bracket/bolt/track arm bushing(s) to exaberate the DW. The bracket/bolt/track arm bushing(s) assembly must have a very tight tolerance and the bolts must keep properly torqued. Loose or improperly torqued track bar mounting bolts are typically what starts DW in the first place. He may need to weld some grade 8 washers or machined gussets to the track arm mounts if there is any play or elongation of the bolt holes.
Remember, both of these fixes must be completed at the same time. If he just does one or the other, he will likely still have DW and will start the wear/tear issue all over again. DW is a vicious cycle, but it can be fixed.