Hmm...
Unarmed Attacks are a weapon. I punch you in the face, my weapon is my fist. (

Rigg's was a Lethal Weapon.

)
As for Spring Attack, cannot Grapple/Spring Attack due to RAW.
(1) Cannot Spring Attack because once you are grappled, you cannot move a opponent because you have to perform a Standard Action, which you can't do multiple of, since the Grapple was the Standard Action of the Round. You need a new Standard Action and a Roll to Move in a Grapple.
THAT SAID...
This wasn't a REAL Grapple. This seems more to the point of Running Past and Pushing/Tackle/grab out of the way. I would have allowed this WITHOUT a second's hesitation because...it is heroic and cool and valiant and ... Heroic! The ONLY rule that needs to be followed in this or any other game is ignore any rule to have fun!
What this really should have been was a BULL RUSH (so no Spring Attack would be needed). As he is hitting the Helpless (not Prone) PC and moving him away. I just would have given the Rushing Opponent either no STR resistance roll or a -5 to a "Take 10" roll.
In any event, I would have given the Cube a AoO, unless the attacker had a Improved version that negated some types of AoO possibilities. If it was something that had multiple AoO's I would have given it possibly TWO (okay, maybe three). One for entering threatened zone, one for grabbing the helpless dude and one for running out/away from threatened zone.