[instantneedles]

Not pushing OP or his gf to learn mandarin, but just wanted to note that the cantonese dialect involves alot of slang/informal phrases that might make it challenging for some to pick up on right off the bat without laying a solid foundation. I'm not saying mandarin doesn't have slang, but teaching cantonese WITHOUT slang is that much harder because cantonese is based off of slang.

An example to illustrate my point:

In english, the expression "how are you" is a common one and is often how you'll hear people start a conversation.

It is formal and at the same time, is used frequently.

In cantonese, the expression "lay ho ma, (aka ni hao ma in mandarin)" is a formal way of saying "how are you" in english. Yet how often is it that you'll hear people say that? Once in a while, but not often.

to my understanding, reading/writing is pretty similar between the two, with subtleties that reflect on the way it is spoken in the respective dialects

Like others have mentioned, being in an environment where others speak the language is one of the best ways to learn. Since she can't allocate time to be in a classroom environment where she can learn from a teacher in person, it will be hard. There's only so much you can learn and absorb from reading books and listening to tapes.

Be like skinny and actually reside in the city for a few months and you'll be amazed at how much you can soak up from just experiencing the culture itself. Remember, humans learn best from approaching things from exposure to multiple mediums and channels, and while reading and listening can be great, you're missing out on the main purpose of language - APPLICATION

Interest is a big one as well. Without it, the most you'll pick up are a few popular catch phrases