Standing up from prone and AoO

[NunoM]

This actually happened in the last session...

A foe was standing up from prone within threat range of my trusty glaive, so i take my due AoO... The thing is: the DM applied the full AC to the guy as if he wasn't prone.

I argued about this, using the argument: "If i can't make a trip attack as an AoO to a guy standing up from prone, because he's still considered prone, why doesn't the penalty for being prone still apply to him?"
Response: "house rule" and the rest of the table agreed, because the same ruling apparently has been used to the benefit of the PCs in the past.

This closed the argument right there, but i still wonder... Was i right in my reasoning? What do you guys say?

BTW, whatever you say won't affect the game now. A house rule is a house rule, and it stays, so i won't argue it at the table anymore... but i just want to be sure on the actual ruling for this in the game.

Thanks!

[Kethrian]

Your reasoning was correct.  An opponent still counts as prone when they provoke the AoO for standing, which includes the -4 AC penalty.  The reason is because the AoO interrupts the provoking action, acting before it's completed, so the foe would still be on the ground at the time.  After the AoO, he can complete his action, which means he then stands up.

[kitep]

Ah, but can you trip a guy who's already prone?

ie, if the AOO involves a (successful) trip attack, does the guy now have to use a 2nd move action to stand up?

[Kethrian]

No you can't.  I believe there was a Sage Advice that covered that, but also, prone is not a stackable status.

[NunoM]

Aye... It's here.

[kitep]

That was a good article.  I knew about the -4 AC while prone, I didn't know about the -4 to-hit or that crawling 5' provokes an AOO.  I've also always allowed the "tripping" of magically flying creatures in my game, assuming it put them off-balance and while it didn't knock them to the ground, they would have to use a move action to recover.  Fun read.

[NunoM]

Thanks for the input, guys!