Interesting to read everyone's thoughts so far but a little surprised that so many people believe that a maul is formed by the a team mate of the ball carrier grabbing/pushing/pulling a defender.
Let's be technical;
A maul is formed when the ball carrier is HELD by an opponent and a team mate of the ball carrier BINDS to the ball carrier.
Let's be practical;
If we have a Red ball carrier and a Blue defender grabs him and then a Red team mate moves in and grabs both/either/or (without specifically running around the ball carrier to concentrate solely on the defender), we let it breathe for a second or two to see if the whole thing falls over, for a tackle situation, or if it stays up (usually the 2nd red player will be driving the red ball carrier and blue defender towards the blue DBL), in which case the ref will likely call maul. It is a dynamic situation and whilst technically it may not actually be a maul (red team mate bound to ball carrier), as long as the 2nd red player looks like he's attempting to bind/wrap his arms around the original 2 players, most referees will call "maul".
You see "non-mauls" form in juniors where a red ball carrier is surrounded by 3, 4 or 5 etc blue defenders and more red team mates attempt to get to the ball carrier but can only make contact with the blue defenders (i.e. they can't physically get to their mate with the ball). 99% of the time, the ref will call maul.
Back to the original post;
In the OP, no maul has been formed by the action of the 2nd red player. He makes NO attempt to bind to his team mate, AKA the ball carrier, and instead makes a B-line for the blue defender and pushes him away from the ball carrier.
Seen this tactic before? Yes, in American Football.
Would we allow a couple of team mates of a player with the ball to shadow his run and drag would-be tacklers off of him as he makes his merry way up field? I think not.
The OP situation is not that uncommon at the elite level and it is frustrating to see a defender, who is legally attempting to tackle or hold a ball call carrier, being grabbed and illegally pulled away by a team mate of the ball carrier.
It is playing an opponent without the ball.
10.4(f) Playing an opponent without the ball. Except in a scrum, ruck or maul, a player who is not in possession of the ball must not hold, push or obstruct an opponent not carrying the ball.
Sanction: Penalty kick