Its not such a dumb thing, seeing as the boss had big buds and ganja like lots of P during flower - phosphoric acid - it kinda makes sense.

If he's putting say 1 can in a 3 gallon bucket (or even around the plant if its outdoors) can't see it will do much harm. Anything can kill a plant in excess, even pure water.

btw, if you take 100ml (approx 105g) of water and add 25g sugar (heat it up to dissolve), you will still have 100ml liquid but now it weighs 130g so you could say it's 'only 75% water" and be correct.