Mint looks quite similar, and if she asks “Microsoft applied an update and now the start menu looks different.”
If all she does is browse the internet and read emails, she’ll never know the difference. You could even set up the splash screen to display the Windows logo or just disable it all together.
You can always compile from scratch, compare the checksums or use the version you compiled. In projects this large people usually do this, and there’s a certain level of trust that these checks have been performed.