Foliager feeding in flower?

depends how high the humidity is in your grow room but in general its not good practise to mist or spray them once they have gone into flowering as it increases the chance of bud mold and leaf fungus such as powdery/downey mildew and spraying fertiliser on the bud will probably cause a nasty flavour.

the grow shop is right

You should stop two weeks into flowering is the rule but I feed with maxi crop seaweed till week 4. I try not to get to much on the buds. But never had a complaint about the taste. And mold is a problem I don't have a lot of. But everything is up too you. Just my opinion. Good luck.