I do not know the rules inside and out, but I think there are 3 parts of the play that complicates the matter.
1. Did Lewis fumble the ball. I would say yes. The defender's action causes the ball to come lose from Lewis.
2. Did Lewis regain control. This is where I do not know the rules. I thought Lewis pinned the ball to his thigh pretty good, while he was going down. I could be wrong, but that is what I thought.
3. When is the play called dead. I guess that depends on #2. IF he had control (as in this case the ball is between his hand and thigh), I would assume that when his knees hit the ground, he would be down by contact. Similarly, I thought the ground caused the ball to move. Does the ground 'aiding' in a fumble at that point have no bearing on the decision?
I guess if the argument is that #2, Lewis never regained control, then the ground would not make the play dead and it would be a live ball until Lewis establishes control for a third time?