Seems like she needs some help to get it in order initially, and then help getting a system together to maintain them.
Get her to put a bit of a vision board together, what would her room look like in her dreams? What do you need it to be (floor clear for hoovering, clear surface to dust, no dishes or food upstairs).
Watch a few Netflix organisation shows (eg The Home Edit, Tidying Up with Marie Condo, home revamp shows etc).
Then work through stuff together - set the parameters for what needs to go, how much can stay, what systems and stuff she'll need in place.
Then get on with a huge declutter - work through each category together, agree how much of what needs to stay or go, and move everything in the category out of her room and lay it out somewhere to go through, donate and bin other stuff. Repeat til you've been through the entire room. Then deep clean (they may be something you do without her), and put every thing back sensibly. If she needs an incentive, redecorate whilst it's empty, and get new furniture to help with the organisation. Then everyone needs to enforce the rules on no food, keeping tidy etc. Probably need a bit of a carrot and stick method, so maybe something like 'you'll clean it if it's tidy, if it's not tidy she looses her phone til it is'. And agree what tidy looks like beforehand, and teach her how to maintain that standard.Maybe a checklist of what needs done would be useful.