I stand to be corrected but reading between the lines he has spun the captive nut holding the inboard nut of the upper arm, nothing more.
This simply requires the removal of the subframe (or maybe not
if you are prepared to enlarge the hole just below this nut big enough to be able to get a grip on it
)
Anyway, once the subframe is removed the nut is accessible to re-secure/replace.
It is not an overly difficult job for any competent mechanic.