I used to live in a place where there were a lot of college students and a lot of musicians, and a lot of older generation original hippies, and everyone there called each other dude. It's just a way of saying "buddy."
I'm a woman and once in the middle of a conversation, I got called dude by a male musician acquaintance who then instantly apologized and asked if that was offensive. I laughed and said it wasn't the least bit offensive!
If anything, I was flattered to be included in the circle of friends whom he called dude. I was glad to be thought of as one of the gang, which were mostly musical, artistic, college age, laid-back males.
A similar one is "man." Also used a lot by musicians, college students, hippies, etc. Women I knew would say to each other "I hear where you're coming from, man" even to another woman.
"Man" and "dude" have kind of transcended their gender boundary as something to insert into a sentence spoken to someone you feel familiar and comfortable with, whatever gender.