The player had his hands on the ball so in this situation I would deem him to have possession (and is the 'ball carrier').
Law 7 makes no mention of being able to use your feet to get the ball from the ball carrier. Law 10.4 (e) states that you cannot tackle a ball carrier dangerously. Also note that it says tackle the ball carrier, and not take possession of the ball.
For me, in this situation, it is an illegal move as it is dangerous, and carries a high risk of injury.
Rugby is also a game for players on their feet, so I can't see how you can allow someone to recklessly dive in with their feet on the ground such that they're not in control of their body. Even if this was deemed to be a tackle, then law 14.1 (d) would come into effect:
(d) A player on the ground must not tackle or attempt to tackle an opponent.
Sanction: Penalty kick
If this occurred in Association Football, then the likelihood would be that a YC is shown as it is reckless play. If however they made contact with the ball first, and no (or minimal) contact with the player in possession subsequently, then it would be a fair challenge.
However in rugby, I would think that if a player has possession of the ball, and is therefore the ball carrier, then they must be tackled properly, with body to body contact, and a clear attempt to grasp. For me, this 'slide' doesn't constitute a tackle, and as such is not permissible.
In this sort of situation, I think the issue is one about timing. If you get it right and make contact with the ball before your opponent makes contact with the ball, you save a definite try. If you get it wrong the try is scored and you're most likely in the cooler for at least ten minutes.