I recently installed Kubuntu on my wife's laptop. She's used to Windows, and I want to make her transition as seamless as possible. Plasma is nice as is, and it felt similar enough to Windows for her to be happy with. One thing she was concerned about was her ability to use MSOffice on Linux, as she's required to use it for her school assignments. My suggestion was that she try using Office 365 online for a little while, and if she didn't like it we would look into getting her a new computer. I would like to make it feel like she's just opening another application when she opens up Office 365 online. Is there a way to write a script and associate it with a desktop/taskbar icon or start menu selection to have a browser open up signed in and directly into Excel/Word/whatever she needs, and minimize the browser decorations (i.e. tab area, url-area etc.) The hope is to get her as close to feeling like she's using the native desktop application as possible.
SnapOverflow
You can use the Office365WebDesktop Snap.
However, consider moving away from Microsoft products to prevent vendor lock-in. Several good alternatives are available, including Onlyoffice (FOSS), LibreOffice (FOSS, installed by default in Ubuntu), FreeOffice (proprietary) and WPS Office (proprietary). Some of these have excellent compatibility with MS file formats.
"Web apps" can "integrate" web sites into the system by opening them in a separate browser window with browser decorations removed, and with an independent icon in the taskbar.
I use Lubuntu, not sure how this looks on Kubuntu. If you have a OneDrive account, you can use the official .deb of Microsoft Teams to manage Office documents. Once logged in, click on "Files" on the left (it may be under the ...), and under "Cloud Storage" there should be the "OneDrive" folder, where you can create and edit documents. The UI is almost identical to the Desktop version:
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!