There is/was a show on TLC where an expert would go into a cluttered house and help clean up a few rooms, and then redecorate it for the family. I think the show was called CleanSweep.
The declutter would go like this:
Empty out the room, into the yard and divide everything into 3 piles: Keep, Donate/Sell, Trash. For the Keep stuff, he would only allow enough stuff back into the room that fit on a certain sized tarp, including the furniture. But just seeing the amount of stuff these folks had accumulated into their yard was shocking enough for most!
Then he would work with the family to go through their personal items. If it was 30 pairs of shoes, he wouldn’t say pick out the ones you want to keep. They would’ve kept 25 and tossed 5. Instead he’ll say you get to keep 8 pairs… which ones do you like best? He did the same with clothing, pictures, momentos, craft items, collections….
Giving yourself a limit works better than keeping “what you want”.
Also when choosing what goes back in. Decide what is wanted on each shelf and in each drawer, then toss what is leftover. Do not start with a bunch of items trying to figure out how to fit them back in. I do this a lot when I’ve accumulated too many knick knacks on a shelf.
The team would then have a yard sale and sell all the extra stuff. The family would keep the money. This was also some incentive to get rid of things. We do this with our DS when we have a summer yard sale. It helps him to let go of more toys if he gets to keep some of the money to buy future items.
And then of course have a redecorated room gives the family a fresh start. Is there a way you could paint her room or re arrange the furniture for a new look?
To help this not happen again (and not that I should talk, my DS’s room is a cluttered mess) is to decide where everything goes and label stuff. If kids do not know where something lives, they won’t even know how to clean it up when asked “clean up their room”.
Maybe you can reward her in a month of clean room with a friend to come over for a sleepover?