As a standard action, a thomil of size Small or larger may attempt to envelop an opponent at least one size category smaller than itself. It cannot make a slam attack on the round it engulfs prey. The thomil merely has to move over the opponents, affecting as many as it can cover. Opponents can make opportunity attacks against the thomil, but if they do, they are not entitled to a saving throw. Those who do not attempt opportunity attacks must succeed at a Reflex save (DC 10 + 1/2 thomil's Hit Dice + thomil's Constitution modifier) or be engulfed;
Freedom of movement allows a player to move freely and automatically escape grapple. Already it seems somewhat contradictory, because the conditions for becoming engulfed are not avoided. The Thomil is the one moving, and no grapple checks have been made yet.
But engulf also states
Engulfed creatures are considered grappled and trapped within the thomil's body
The engulfed creature can instead attempt an Escape Artist check (DC 15 + 1/2 thomil's Hit Dice + thomil's Constitution modifier)
So these clauses suggest that a character using Freedom of Movement can escape being engulfed with absolutely no trouble.
Is there are more clear ruling than this? Getting engulfed only to immediately escape is a strange way to handle it. It makes more sense to me that the victim would never be engulfed in the first place, but I am worried my players will try to rules-lawyer their way out of it. I want to clear up this ambiguity beforehand so he won't feel too gotcha'd.