Thankyou for the question,
This ruling is probably best answered through a step by step process to ensure transparency.
players X and Y are having a shootout and player X wins by 2 ranks, player Y chooses dude A (who is harrowed) to cover the total casualties, due to his being harrowed this means he would be discarded, player X then chooses to react with Hellstromme. This becomes a replacement effect covered in the rulebook and FAQ by the following statement;
Some Reacts replace the original event with a new event; these can be identified by the use of the
word “instead” within their text. Once a React of this type is played, the original react window is
closed and the replacement event occurs. Additional Reacts can then be played in response to
that new event.
Dude A has already left play at this point and therefore is no longer harrowed (harrowed only works while in play) he therefore is Aced.
TLDR; Harrowed only works whilst in play, Hellstromme says after an opposing dude is discarded or aced.