Dash is necessary if a predicate and a subject are both nouns. Вона is not a noun, but a pronoun, so rules don't require a dash here. It (dash) can be put in sentences like this one for emphasis (it is her who is my friend)
No. "Подруга" is just a female friend. (Like a 'best buddy'. For "girlfriend", as in "My girlfriend." I'd say, "Моя дівчина."