New rider, and new passenger, I'm sure as experience comes she will learn to relax more, and get her comfort on the bike.
Many a time I have had new passengers complain of this, often it takes a few runs to get them to relax, making more room for the lass will no doubt help, maybe a sheepskin cover for the seat, I have one that is designed for a horse riding saddle, and makes a big difference on a long ride, but no good In rain...