Personally, if I was trying to help someone from a liberal arts background, I would start them with some of the "popular" books out there until they had some basic background in the concepts and a real desire to dive deeper. The "popular" math books are easier to stay motivated reading, and I read a lot of them as a break from text books when I was working on my undergrad in Mathematics.
I am a fan of many of John Allen Paulos books like "Mathematics and Humor". "From Zero to Infinity" is rigorous, yet written for the non-mathematician (or at least beginner) in an informal, easily read style. "Beyond Numeracy" is a very approachable book with short sections on a lot of mathematical topics and targeted at the undergrad level.
For something slightly more in depth and rigorous, she may want to look at "Chapter Zero" which goes through a lot of the stuff that people should have learned by High School, but almost never did. It is targetted at a High School level audiance and is easy to read, but unlike my other recommendations it is a text book and can be a bit dry and can require some motivation to get through.
Finally, I have to mention "Visual Complex Analysis." It is one of the most approachable and entertaining books on serious mathematics I have ever used, and it is easy to just read selected chapters. (That is what I did, I still haven't finished going through it cover to cover, and used it as a supplement to my text books). But that one is about serious mathematics and is targeted at advanced undergrads in heavily mathematical fields.