When player can't see invisible and throws a potion of visibility at invisible stalker, the invisible stalker will still be referred as "someone" or "something" (even if named), like in the following messages:
"This is someone. She is hostile. Something is not injured."
"Someone misses you."
"You kick someone and moderately hurt it. Ambaretelydes, the great blue wyrm, attacks something. Something is killed!

If player can see invisible, the monster is still displayed as invisible stalker.