Stick to a consistent bathing routine. Make it the same time the person previously bathed (first thing in the morning, right before bed).
Don’t remind or even mention how long it’s been since the last cleanup. Instead of arguing, proceed with bath preparations.
Don’t ask, “Did you shower?” or “Would you like to shower now?” Get everything ready and invite the person in: “Look, your bath is ready. I know how you love your evening bath”.
Try leading the person to the bath unexpectedly, on your way to doing something else. Or lead the way to the bathroom but without talking about a bath
Have everything ready so you don’t leave the person alone, as she may abandon the idea..
Don’t worry about a full daily bath or shower. Bathe weekly and “top and tail” (face, genitals) as best you can the other days.