A buddy of mine was playing at FNM last night, and attempted to Path to Exile a Slippery Bogle (facepalm). His opponent said that since he had already cast the spell, he had to pick a legal target. That's incorrect, right? What about at a higher REL?
That’s incorrect at all levels of play.
If a player takes an illegal action that’s caught right away, such as casting a spell on an illegal target, then the action is rewound. The player isn’t forced to choose a legal target if they don’t want to (unless it’s a triggered ability or something.)
Magic isn’t a game of gotcha.