A 139: They provoke an attack of opportunity as they start the action, but before they complete the action.

Explanation: An attack of opportunity "interrupts" the normal sequence of combat.

Quote Originally Posted by Attack of Opportunity
If an attack of opportunity is provoked, immediately resolve the attack of opportunity, then continue with the next characterís turn (or complete the current turn, if the attack of opportunity was provoked in the midst of a characterís turn).
There are many actions that provoke an attack of opportunity, which include standard actions, move equivalent actions, or full round actions. Check the tables on the SRD or the PHB for details on which do and which don't.

The opponent provoking must begin the action, but cannot yet have completed it. We know this because someone casting a spell can have their spell lost if they fail a concentration check, and a player moving cannot have already moved out of range of the attack.

The provoker is still prone while attempting to stand, and while the character making the attack of opportunity is free to attempt to trip, the provoker is already prone and thus the attempt should have no effect. After receiving the attack of opportunity, the individual is free to stand with no further impact.