It's one treatment per passenger. And sadly you can't donate your slot either, if you felt like it.

AFAIK there's no manicure treatment. I can't see how they'd be able to do anything other than a quick polish in 15 mins anyway. You could offer her the best of both worlds and request the hand massage though

Seriously, the treatments are a bit of fun, but they're more like taster sessions than real spa experiences.

EDIT: Ooops, I stand corrected. There is indeed a Jet Set Power Polish option. Still think 15 mins is a bit tight for a decent job though.