Wow!
UPDATE:
I took ruby into our bathroom (its outside, don't worry, and its just the loo and an old sink so we won't use it for ourselves), which mainly has things like horse worming treatment, dog shampoo, etc.
I filled up the sink with water, not for ruby, and held her upside down, continuing to pull off each and every (hopefully) maggot. I did all around and on her bum, but had to dig a bit deeper to find a small patch of them. I picked them all off, and placed them in the water which ad some alcohol in it thanks to some I found in the bathroom.
SO after I picked hopefully all of the maggots off (I'll recheck after school on tomorrow/monday) I saw her bum was quite nice, clean and pink and the prolapse was quite pinkish red, but no bleeding luckily. So after it was complete cleaned, nice and maggot free by the looks of it (I looked into her bum as far as I could before my tweezers saw that the flesh was normal and didn't look at all infected and Ruby was quite fine with this inspection), We found some of mums oils that she'd used on previous birds we'd saved, and it had cleaned and helped their wounds so we coated it on her bum and surrounding skin. And I gently coaxed the prolapse inside, and she perked up a bit and her bottom skin kept it in and she looked much more lively and walked a bit.
So I put some hay into a box and put her in and placed her next to me inside, with water and food, and we'll keep a close eye on her.
Anything else I should do?