I'm so sorry. I honestly think the grief when I lost my first horse aged 15 was the worst I've ever felt, certainly the most intense anyway. (I can't say that in real life as I've lost relatives etc and people will think I'm bonkers
). His was colic too and very sudden, it's really really hard. I honestly cried my heart out for days and I still miss him many, many years later. There's nothing like the bond you have with your first horse.
I'd indulge her for a few days as others have said, let her cry as much as she wants, allow her to stay off school etc, lots of hugs and comforting TV and nice food etc. Can you get some of his mane/tail hair and have it made into a keepsake, I still have the locket with my old boy's hair my Mum had made for me?
Then in the more medium term, getting back to routine and being busy with school etc and friends as soon as she's up to it is probably best for her. I'd probably encourage her to get back to riding as soon as possible, offer some low pressure lessons or a nice hack or similar. Reassure her that she's not being disloyal - when I lost my boy I dramatically swore (and would brook no argument whatsoever) that I'd never ever ride again, because no other horse could ever match up to him (that part was mainly true but the never riding again certainly wasn't
). A few weeks after we lost him, someone on the yard "conveniently" had a really nice older arab boy coming back into work after an injury and "begged me as a favour" to help them get him fit again, so I pretty much got tricked against my will (oh the gullibility of youth!) into enjoying myself riding and throwing myself into a 'project' which did me the world of good. If something similar comes up I'd encourage her to give it a go.
Then longer term I'd be guided by her as to when/whether she wants another horse of her own. I got another horse probably about 6 months later when a nice young Irish type happened to come up for sale on the yard, it helped that he was a completely different type from my old boy so it never felt like I was 'replacing' him. I was ready by then but I wouldn't push her into it too soon...
But really for now just take a day at the time and do what you need to help her (and yourself) through this 