DM Needing Help - Ally hit by blast, now susceptible to triggered secondary attack?

[Dakaran]

I'm presently DMing a PBP game where our controller cast Grasping Tide (a blast) that among hitting her enemies hit one of the other PCs. When Grasping Tide hits a target, until the end of that target's next turn if they move out of the blast's effect zone they will be hit by a secondary attack (opportunity attack). The PC that is playing our controller has asked if they can just choose not to take the opportunity attack against the fellow PC if they choose to move out of the blast's effect zone. Can someone help me with the rules on this one? Can our PC choose to not allow the triggered secondary attack to go off on a target?

Thanks!
Dakaran

[ninja_penguin]

Depends. Does it say 'if the targets leave the area you may perform the following attack' or 'a target that leaves the area suffers the following attack'.

Or if it's an opportunity action, etc. etc.

[Dakaran]

Mmm... I don't know how I overlooked it, but it does say "can use". I think that answers my questions. Thanks ninja_penguin!

Grasping Tide (Druid) Druid At-will lvl 1
A vortex of water appears amidst your foes, drawing them into its grasp
At Will* Impliment, Primal
Standard Action; Area burst 1 within 10 squares
Target: Each Creature in Burst
Attack: Wisdom vs Fortitude
Hit: 1d6+ Wisdom mod (4) damage. Until end of your next turn, if the target leaves the burst's area of effect, you can use an opportunity action to make a second attack against it.

Increase damage to 2d6+wisdom modifier (4) at 21st level
Secondary Attack (Granted by opportunity action): Wisdom vs Reflex
Damage: You knock the target prone

[Duos Greanleef]

*le snip*
Hit: 1d6+ Wisdom mod (4) damage. Until end of your next turn, if the target leaves the burst's area of effect, you can use an opportunity action to make a second attack against it.
*le snip*

It looks like he doesn't NEED to take the action to make the attack.