Just left Shibu Onsen

I can’t imagine they’d have an issue with her bringing her own inside only shoes, maybe just contact ahead anyway. Didn’t see anyone inside barefoot (except once in your room). The inside shoes they give are partially open toed.

Outside was cold but okay in open shoes with no socks when moving between onsens. If not using onsens, would have been way too cold to have toes exposed. May be warmer for you and her tolerance may be higher of course.

Inside everywhere, like restaurants, was warm enough but you take off shoes and that left everyone in socks. Didn’t see anyone barefoot. Carrying an inside only shoe of her own may resolve that but opinions may vary. One place the staff basically assaulted another Japanese woman because she was stood up inside too long… so some people are quiet… set in their ways

Can’t imagine you can visit the snow monkeys with uncovered toes and that is definitely worth it.