I was recently shopping for a bike and rode 29ers and 27.5s. The 29ers do roll over stuff better, but the 27.5 felt better to me at 5-8. Both bike I rode were Anthems. I ended up with the Anthem SX 27.5 and I am enjoying it! Still hauls the mail over roots and rock gardens and I feel balanced on it.
For me the biggest difference was going from 26 to the 27.5. On paper, it didn’t look like much of a change, but it is.
Have her try several out!