KSG/Kel-Tec did nothing wrong. The user attached an unsafe accessory, and that's the cause.
I've attached 2 foregrips on my KSG to try out: Magpul AFG2 (Angled Foregrip) and Magpul RVG (Rail Vertical Grip)
The AFG uses one screw inserted in the rail slot to hold it and prevent sliding. The RVG has 2 screws in the slots.
The ProMag has a disc and a plastic nipple that keeps the grip stationary.
The real problem is, most foregrips for AR-15 style rifles are not going to be used to pickup, run with, and generally handle the rifle using just the foregrip. The stress on the foregrip is pretty light.
On the KSG, the grip takes a lot of stress as it's used to pull and push the slide.
There are plenty of blogs, forums and articles on the KSG's polymer lower rail and the types of grips to use with it. There's no reason to be using the ProMag grip unless the owner just didn't do any research.
I was concerned about the proximity of the forward hand to the muzzle, so I opted for the compact Viridian laser/light on the front of the lower rail which acts as a hand stop, and the vertical Magpul grip because it's attached with 2 screws (plus the AFG is too long with the Viridian sight).
Sorry about his hand, but using the wrong grip was his fault. If you don't know what you're doing, you need to ask a dealer, gunsmith, or other professional for advise.