Well here's something interesting:
Below Waist
ARTICLE 2. a. A block below the waist is a block in which the force of the initial contact is below the waist of an opponent who has one or both feet on the ground. When in question, the contact is below the waist (Rule 9-1-6).
b.
A blocker who makes contact above the waist and then slides below the waist has not blocked below the waist. If the blocker first contacts the opposing player’s hands at the waist or above, it is a legal “above the waist’’ block (Rule 9-1-6).
According to this, I believe the block was legal because it started above the waist before the OL fell down. Anyone else have input?
EDIT: I should do more research before I start spouting off.
Here's some more relevant info:
Blocking Below the Waist
ARTICLE 6. a. Team A prior to a change of team possession:.
1. The following Team A players may legally block below the waist inside the tackle box until they leave the tackle box or until the ball has left the tackle box: (a) players on the line of scrimmage completely inside the tackle box and (b) stationary backs who are at least partially inside the tackle box and at least partially inside the frame of the body of the second lineman from the snapper. (A.R. 9-1-6-V)
2. Except as in paragraph 3 (below), players not covered in paragraph 1 (above)
while the ball is still in the tackle box, and all players after the ball has left the tackle box, are allowed to block below the waist only if the force of the initial contact is directed from the front. “From the front” is understood to mean within the clock-face region between “10 o’clock and 2 o’clock” forward of the player being blocked. (A.R. 9-1-6-I-II, IV, VII-VIII)
3. Players not covered in paragraph 1 (above) may not block below the waist toward the original position of the ball at the snap until the ball carrier is clearly beyond the neutral zone.
4. Once the ball has left the tackle box a player may not block below the waist toward his own end line. (A.R. 9-1-6-III)
I think the block was legal, but if the official never saw the initial contact then they will call an illegal block below the waist.