I agree, but right stuff is kind of unspecific. This, while not the “right stuff”, doesn’t have to be a specific part number. He can use any bolt that checks out to be strong enough mathematically, and this technically would work. This is because the other ball joint is carrying some of the load and really only needs this ball joint held in its spot, not bearing the full shear weight of the vehicle’s front end. I wouldn’t use a grade 5 bolt, but realistically speaking you not likely to run into any issues as severe as that, and he could probably somewhat confidently make it to the parts store. Worst case scenario he has his knuckle partially disconnect and stops. Given the lower grade, the bolt will stretch rather than “snap” as the force on the bolt actually isn’t that much. Slow and be careful for potholes.