Most electrical devices are built to accommodate a 10v variance in voltage. For example, my camera charger can accomodate 100-240v. It will usually be printed on the device somewhere. I think a hair straightener would work fine no matter what, though it might not get quite as hot as normal if it isn't designed for the voltage difference.
Almost all Japanese outlets are polarized two hole outlets. Ground holes are rare.
To clarify frosty's post, families do not all shower together and she will surely be left to shower by herself. If she doesn't want to share bathwater with the rest of the family, she doesn't have to use the bath, and I'm sure she will be able to opt out of any trips to bathhouses, if there are any. Even hotels that have communal baths as their primary washing place usually provide a few private showers.