A PC in my campaign has an enemy grabbed and prone and is attempting to drown them. It's the enemy's turn. Can the enemy escape while grabbed?
If so can he shift while prone if he succeeds on an escape?
Couple of things:
It's perfectly acceptable to stand up while prone and grabbed (grabbing immobilizes, but immobilization does not prevent standing from prone).
Yes, he can absolutely use acrobatics/athletics to escape, but he'll be prone and as such will not be able to use the free shift.
This makes grabbing and proning an effective strategy as it means the monster needs to consume 2 move actions to be able to move freely.
A smart one will not do so however, as creatures without any melee or close attacks are very rare. Being grabbed gives no penalty to attacks or defenses. So the best tactic for them is to Stand up and attack in the first round, then Escape and attack in the next.