Learn Farsi: هی، تو!

Context is everything! Here, it means 'you'. While تو is heavily used as a colloquial shortcut for در (in) in everyday Tehrani speech, it is also the standard informal pronoun for 'you'. Since it directly follows the exclamation هی (hey) as an isolated phrase, it is definitely the pronoun.

Is shouting هی، تو! as rude or aggressive in Farsi as it is in English?

Yes, it is very direct and often confrontational. You wouldn't use this unless you were angry, scolding someone, or trying to provoke them. To politely get a stranger's attention, you would always say ببخشید (excuse me), and for friends you would just use their name.

Is هی just the English word 'hey' borrowed into Farsi?

They are practically identical in use and sound! While Farsi has its own traditional exclamations to grab attention from a distance like آهای, the word هی is widely understood and used in modern spoken Farsi, especially in casual situations or when expressing annoyance.