My friend's mom had funky toenails. We surprised her with a pedicure. She had no idea, we took a group of us girls so we could pressure her and she could not say no. My friend also lied and told her it was prepaid and she'd lose the money if she didn't do it.
So...she got the pedicure. Her feet soaked in the water with the jets, nails nicely trimmed, feet massaged. She ended up enjoying it.
I agree with the shower chair/bench. Call it a "spa chair". Be blunt, tell her she stinks and other people noticed it. Set rules like no going in the car without a shower. She can't smell herself.
Another friend of mine, his Mom got yeast infections under her breasts from not showering regularly.

