Sadly
@Kool ******
that totally and 100% NOT correct :(
A visa is stamped used when you enter BUT the 90 day entry stamp you get from the visa is good and can't be cancelled even if the reason you got the visa changes once you're here.
They are allowed to stay for the 90 day stamp they got when they entered the country whether they worked a single day here or not. That's how entry stamps from VISAS work.
You're thinking of an extension of stay