I have the predecessor to this jacket and LOVE IT! I actually have damaged mine (long story) and have now asked for this one again this year. A small would be perfect for her as they are fairly true to size and she wants a bit of room for layering underneath. If not for that I'd say get an XS...but you want a little room. This is the only windstopper soft shell I have found that has pit zips, which I have found a necessary. I use this jacket ice climbing and on backcountry skiing...it is my "go to" jacket and can't say enough good things about it. I have the light blue and find that it gets dirty really easily, though, even though the color is gorgeous. I'm going with the green this time but it will probably get dirty too. The advantage is that you can wash it (in TechWash of course), but if that is a real concern (if she is as active as I am) go with black.