I’ll limit recommendations to the last decade or two.
If she’s looking for an amazing and influential work that embodies the form, Leviathan Wakes is it.
If she’s looking for a light read with space opera feel, The Collapsing Empire would work.
If she wants high literature, Ancillary Justice is spectacular.
If she likes a heavy dose of anthropology, A Memory Called Empire is very good.
If she likes weird shit, Ninefox Gambit is space opera with suuuper fucked up apocalyptic math.
If she’s interested in what space opera looks like through the lens of modern Chinese culture, The Three Body Problem is the way to go.
Wikipedia has a list of examples that could be helpful.