How to speak girl

My friend and I were talking last night how BYU needs to teach a class on "How to speak girl". The following story will explain why. He wanted to ask a girl out, so he texted her (yes, I know that is generally frowned upon, but that's his method) and asked "What are you doing Friday night?"
    If you are not a guy, this in fact means "I'm going to ask you out pretty soon but I want to see if you're going to be able to go first before I risk it." To which she answers, "I'm going shopping with my sister," We're both left not knowing what that means. I mean more than the fact that she's going shopping. Does that mean  "I'm too busy to go with you that night, or ever for that matter", or is it "Alas! I have some plans I am not able to cancel! Mayaps another day, my love," or does it mean "I kinda have plans but they might be open to change,"? Way too complicated for us (or maybe we're just overthinking this, also a possibility). The conversation between them continued, and so he still asked her out for Friday, figuring he could see if she might be willing to change her plans. She ended up accepting and now they're set up to go out this Friday (so apparently "I'm going shopping with my sister," means "try harder, stupid". Good to know. I'm going to make a dictionary of this kind of stuff, it'll sell millions.