It's casual speech, and therefore casual with the details. ;) Of course, do expect an answer like Jura said, if you want to use this!

Had never heard about. I know for sure you can say "hey guys",being addressed to group of people ,regardless whether they are male or female, but only in plural form.I thought if you have addressed lady as a man, would be considered as very offensive and vulgar behavour in any language.

A lot of people say "hey you guys/ hey man" even though the group of people are all women. It's an Americanism. It's up to women to stand up for themselves and say, "I don't see any guys/men here, do you?"

same as true as with the term "dude". although "man" and "dude" pertain to a man, nowadays, they became a standard expression among male and female. :)

Here, "man" is informal word to express surprise.