42x17 oughta be ok, if she isn't riding much now then there's no reason that she should even need to be able to spin it at a high cadence right away anyway. She can tool around very comfortably at that ratio with a slow peaceful cadence while she gets the feel of it. My lady is riding 44x16 on her commuting fixed gear, doesn't go too fast and as a retired messer she doesn't care too either, peaceful cruising or efficient gettin there pace is what she prefers now. Hell I'm 48x16 in the winter, which feels slow after a summer of a road bike that never leaves it's 54 in front, but it does feel nice to be fixed again everyday.