Straight up tell her she stinks- it might sound/be offensive, but most normal people don’t like stinking lol. You could also suggest she try a normal bath or a sponge bath! Maybe she hates how loud the shower is or has other sensory issues. (In case you didn’t know, a sponge bath is where you take a cloth, typically a thinner bath cloth, and wet it with some water and soap, then clean your body with it. It’s a good solution to keep yourself generally clean without running water.)