Without hearing the actual exchange it's hard to pin down what the context is. Something sounds funny here -- like it should have been "she won't go for it" instead of "she won't go for you".
Regardless, "won't go for" does not mean attack and rarely if ever. It means she won't be convinced to believe you or your idea.
"Her heart isn't in it" means she is not emotionally connected at this time and can not connect with the person or idea regardless of their intent.
We often say this when someone is mourning (after a death) -- for example, her husband just died and another man wants to go to dinner with her. Her heart is not in it -- she's emotionally not ready to think about dating.
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