1) While grappled, the foe is unable to perform any maneuvers other than to attempt to break/wriggle free.
2) That particular critical also says that foe is stunned 1 round and at -25 for three rounds.
3) Other criticals on that table do specify "while held", this critical (the G crit, (29-32)) is one of several that do not specify such, so there is no reason to think that the minus is only "while held", especially since there is room to include those two words in the critical entry.
Thus, you can conclude the following:
1) Foe cannot perform other actions except to attempt to break free of the grapple while held.
2) The penalty (-25) lasts for 3 rounds AFTER the 1 round of stun (-50, not allowed to attack) has completed.
3) If the character has broken free during the round of stun (he is allowed to try, but gets the -50 from being stunned), then he may make attacks during the 3 rounds in which he receives the -25.