How to adjust a monster's / NPC's CR due to nerfing?

[LeleyX]

I was wondering if there are any guidelines on how to downgrade a NPC, for instance he's sickened, how much lower would the CR go? Is there a list or guide somewhere that I could take a gander?

The specific example I have is this:
I'm running a Rise of the Runelords campaign, last year we had finished the 2nd AP with a TPK (or close enough, I decided the lamia matriarch would just capture the whole lot of them for future ritual sacrifice, thus giving them a chance to escape).
Between then and now there's been a change in the group, a couple members have left, and there's a new guy coming around. So I offer the new guy a choice, start the next AP with his character already in the dungeon OR try a solo rescue mission, as the lamia matriarch has already fought the party, spending spells and hp, I figure it gives him a better chance to get to here if he takes her on as that.
So, think of a level 8 sorcerer/1 rogue at 1/2 hp and 1/2 spells spent, how would you adjust that CR? (I don't have my books here with but I think her base CR was 9 or 10)

Cheers!

[wond3rbr3ad]

Somewhat of a spoiler... I recently ran the first book of Carrion Crown which has the players fight someone in a similar situation; a normally CR 9 wizard doesn't have access to all of his spells so he's written as a CR 6. If you have access to this book, that would probably be a good reference.

Though, do you really need a CR? You could just award roughly half the XP because of the almost TPK and splitting it into two fights to make it easier. Or, since you're running an AP which almost seems to assume that your players get every piece of xp available, just give the party the full xp when she's defeated.