I'd say between a 155 and 158 would work great and if she's naturally aggressive tend towards the larger sizes, but it will largely depend on what you think your g/f will become. Is she going to hit the mountains a couple times a year, or will she be going more frequently? A smaller board makes learning a bit easier for turn initiation and all that, so if it's going to be used occasionally I'd lean towards a smaller board.
- My little brother's g/f is the same height and weight and loves the 155
- My ex g/f was a bit shorter and lighter, but way more aggressive and she much preferred a 157
Both of the girls above have been riding a few years, and both started on a smaller board that what they ride today (152 and 154 respectively). Both use non-female specific boards and have no problem with the 'male' flex patterns and handling, and this was true when they were beginners because there weren't sex specific boards back then.
Since girl's feet are different, so are their boots and bindings, but I know a lot of women who ride regular bindings with no issues...even men's boots. I think a girl's calf sits a bit lower than a boy's so boots are flared a bit earlier in the upper, but the girls I ride with have a mix of men's and women's boots. She would be around a 9 or 9.5 in men's US sizes so have her try a bunch on and go with what feels best.