The way I've always played it, and I've explained it to my players ahead of time and they're cool with it, is this:
If the creature gets to pick the target and the fighter is a valid target and the enemy targets someone else, the fighter's CC triggers.
If the fighter is specifically excluded from being a valid target (as seems to be the case described by the OP, assuming the original target is specifically excluded from the secondary attack), CC does not trigger, but the attack is still at -2.
Basically, if the fighter can be a target but another target is chosen, CC triggers. If fighter is explicitly excluded, CC does not trigger. Some creatures have multi-attack powers that must target multiple targets. Even if each of those attacks is a separate attack roll, we play that it does not trigger CC so long as the fighter is the first target chosen. The attack is still at -2 (per our house-rules).
Not sure what power the OP was talking about, but our group also differentiates between making an attack, and using an attack roll to determine if someone is hit. In the case described, it sounds like we would count the original attack as "making an attack" and the secondary attack as "collateral damage that you have a chance to avoid". That rarely comes up, but it seems like it would apply in this case.
I know this may not be exactly RAW, but that's the way we generally play and everyone is happy with it. We also examine it on a case-by-case basis, so for other specific cases, everything I said may go right out the window.
