If you want the best compatibility, install Microsoft Office in whatever wine/proton you can get for Kubuntu (plausibly Steam), otherwise I'd recommend she uses LibreOffice.
It's been my experience that LibreOffice has debatably worse stylistic elements for non-scientific use (high-contrast colors, hard edges, etc. by default), but wins out in a few areas and is compatible with all the meaningful features other than Excel Macros (often the only tool available to miserable office workers). I exclusively used LibreOffice (and Google Docs for presentations and simple paper collaboration) during my now-dusty engineering degree, opened and collaborated on non-free documents, worked with formulas, graphs, presentations, largely without issue even the most basic laptop. LibreOffice is also debatably more ethical to use than a closed-source tool, but I don't think you should make that a priority if you're not in a position to.
The PC-native Microsoft tools tend to work well for what they are and have a lot of visual niceties that are well-welcome, but the o365 versions are excruciating to work with, with egregiously missing key features and bizarre display quirks. This is especially clear when doing something like drawing and labeling a chart from shapes and arrows in o365 PowerPoint or deleting rows in o365 Excel (this frequently causes an improper display position of in-sheet vs visual data for rows that were off screen, directly causing errors and wasted time). I've had to put it down and go for a walk for fear of bursting, and jokingly fear for your marriage!
