To clarify the problem, here's the card text for both cards mentioned.

Jack Sparrow: If you discard this card from your Friend Area, choose the highest level Friend Card in play (if there is a tie, choose one of them) and discard it.

Leon: This card cannot be discarded unless it has participated in a battle or challenge.

Consider the following situation:

You've got Leon in your friend area, and that's the only level 4 friend you have. You have some level 1-3s in play, but no Sephiroth/Cloud/Whatever whatsoever.

Your opponent discards Jack Sparrow.

What happens next? Do you just discard the next-highest level card in play, meaning one of your level 3s? Or did Jack Sparrow just get discarded for no reason?

My guess would be that you discard a level 3 Friend.

Cameron

First off, Jack does state to discard the highest level friend in play, so he could very well knock out your lvl 4 friend if your opponent's highest 'n only lvl 4 is Leon, so be careful when you decide to discard Jack.

Second, if your opponent had Leon as his only highest, 'n you've no lvl 4s, Leon's effect pretty much declares that he cannot be target'd by the effect (same concept as when you are playin Gravity. You can't target Leon to save face), 'n so the effect would then goto the next highest level.

Proceed gameplay from there on.