Bro S16 sorry to hear what happened to your friends Iswara windscreen. First of all the, let me share with what I think might be the cause of the problem. The water/stain remover in liquid form is most probably acid based and quite corrosive (very low pH value). The problem with using those acid based water/stain remover is it works well for glass surface that needs to be entirely flat with no scratch marks, pits, etc. So usually it is only safe for side windows and some rear windscreens (no wipers and glass dependent). Running your wipers on the front windscreen will create micro scratches (especially when windscreen is dirty) and gets deeper over time. Those scratches is usually not visible unless inspected properly under bright light. Even sand, dirt, stone, etc hitting your front windscreen over time would create scratches, pits all over the surface. When those acid based products gets into those scratches/crevices it may get trapped in the crevices and etch the glass thus causing the haze. Also those acid based products needs to be washed off immediately and properly after use - better neutralized. Besides the product, most modern cars have laminated glass for the front windscreen which is softer (probably easier to be etched by the acid) than tempered glass (side windows and some rear windscreens). That's why I stay away from those acid based water mark/stain removers.
A better and safer approach would be to try vinegar first which is a light acid to remove those water mark/stains. If it does not work, the safer method would be to use compound/abrasives meant for removing the water mark/stains. I posted an article on removing the watermark safely before on ZTH detailing forum. I've already tried it on a few cars and front windscreen too without any problems. Here is the link:
http://www.zerotohundred.com/newforums/car-detailing-forums/396809-safe-and-effective-waatermark-removal-on-glass.html
That said, I am afraid your friend's car windscreen would need to be polished off using special glass compound/abrasives (like Cerium oxide) and a rotart machine polisher with the right glass polishing pads (felt) in order to restore back the clarity. If the car windscreen isn't too expensive, get it replaced.
My 2 cents...
---------- Post added at 10:13 AM ---------- 6 hour anti-bump limit - Previous post was at 10:11 AM ----------
Bro S16, you can try CarPro CeriGlass kit which you can purchase from Ultimateshine website to see if you can remove the hazing. Not too expensive but no guarantees :)