You may need to move the drive to another computer, or use a USB bootable drive with Linux to access the filesystem. You can recover at least some of her files from the OneDrive folder under her profile, anything that was cached locally. I'm not totally familiar with how much OneDrive will cache since I refuse to use it and having things like Desktop and Documents moved to non-standard locations and not knowing what files you actually have on your computer is stupid. Most likely she wasn't using any special folders besides the usual like Desktop, Documents, Music, etc., and it's not likely she moved any other folders to OneDrive, but you can also look through the system to check for any odd applications that saved files to their own folders. Also check the Downloads folder, since that doesn't sync.
Did the recovery options involving the phone number not give the option to have it call the number and read a code aloud? I went through this a year ago and I can't remember if they stopped doing that.
I went through the same thing with my stepfather's account last year, when my mother's PC started to fail a few years after he died. He had it all set up on what was basically a throw-away account, likely not realizing how Microsoft was tying everything to it. The troubleshooting process ended up basically bricking the PC because Windows decided the BIOS update was a security problem and required the password instead of just the PIN, and we didn't have it. Luckily everything we needed in terms of files was on the computer because OneDrive wasn't syncing those main folders, but I was never able to recover the MS Office license. He also used a landline for the account, but my mother had cancelled that phone number since then anyway, and she had cancelled the Internet service that provided the email account he used for it (he'd had that account for 30 years, before ISPs stopped providing email). Since the MS account was never actually used for anything, none of the recovery questions were answerable like providing subject lines from emails. Despite her having a large booklet of accounts and passwords, none of them matched the MS account, so I just was glad the most important data was accessible.
So you can probably get at least some of her files, but some may be lost, and if she had applications like MS Office tied to the account you won't be able to get those either. Plus of course if she was using that MS account as a real email address (@outlook.com or @hotmail.com) then she'll lose all access to her email, and anything that is tied to it.
Something to consider is copying the profile folders for Chrome or Edge or Firefox and migrating them to another computer. If she had her password saved in the browser, you might be able to retrieve it there. Or maybe try other passwords that she knows or that are in the browser, if there's a chance she re-used them.
