This is a tricky one, as there appears that there could be a conflict in the laws of the game depending on how they are enforced.
I personally believe this will be Not Out, as the ball striking the placed helmet will result in a Dead Ball - as per Law 18.104.22.168 which states:
20.1.1 The ball becomes dead when
22.214.171.124 there is contravention of Law 28.3 (Protective helmets belonging to the fielding side).
Law 28.3 referring to the use of fielding helmets
28.3 Protective helmets belonging to the fielding side
28.3.1 Protective helmets, when not in use by fielders, may not be placed on the ground, above the surface except behind the wicket-keeper and in line with both sets of stumps.
28.3.2 If the ball while in play strikes a helmet, placed as described in 28.3.1
126.96.36.199 the ball shall become dead
However, under Law 33.1 - Caught, it states that a catch taken after hitting an obstruction that is not the boundary (as considered by the umpires) is considered a fair catch:
33.2.2 Furthermore, a catch will be fair if any of the following conditions applies:
188.8.131.52 the ball is caught off an obstruction within the boundary that is not designated a boundary by the umpires.
in which case you could make the case that the decision should be Out.
I guess the argument here is whether the ball is considered Dead immediately after striking the helmet, in which the batter will be Not Out, or if play is allowed to continue and a fair catch is made, then it would be Out. However there does not appear to be a law that states this explicitly.