This is an illegal block. Quoting FIVB rule 14.1.1:
Blocking is the action of players close to the net to intercept the ball
coming from the opponent by reaching higher than the top of the net,
regardless of the height of the ball contact. Only front-row players are
permitted to complete a block, but at the moment of the contact with
the ball, a part of the body must be higher than the top of the net.
and 14.6.2 which lists the faults which can be made while blocking:
A back-row player or a Libero completes a block or participates in a
In particular, note that it is the fact of reaching higher than the top of the net which makes it a block. Whether or not the player jumps is irrelevant.