I think it is grammatical, though it may change the semantics. OTOH: > can one choose to use her time at her own will? changing to: can one choose to use th

I think it is grammatical, though it may change the semantics. OTOH: > can one choose to use her time at her own will? changing to: can one choose to use they time at they own will? is nothing like English. You could of course use "their", but that has a kind of distancing effect.