One tip I picked up from watching that show on TLC, oh, what's it called, with the organizers & the yardsale, hmm...well here's how it goes. Take everything out into the yard (they do the front to sorta' embarrass the people into keeping clean, but the back might be more discreet in your case). make 3 piles: Keep, Trash, Sell. Do a quick sort to go through in 30 minutes to get started, then go through more thouroughly over the next couple of hours. Once the "keep" pile gets manageable, then you can throw out all the trash and have a yardsale with the "sell" pile or send it to the mission. It is a lot easier to clean with all the stuff out of a room.
Good luck! You're a good friend. I had to help one of my friends do this, but she had like 25 years worth of clothing and real attachment issues. It may seem daunting, but I agree with the room by room assessment of things. It will help make it more manageable.