An adult female Japanese friend of mine has started calling me "chan" instead of "san," which I assume means it's OK for me to call her that too. But if she were male, would it be right for me to start calling him "kun"?
She calls you CHAN because she likes you. I assume you are a guy. CHAN is an affectionate term and I have even been called it. If she was a guy you could possibly use KUN but not if she was much older than you. I call some girls older than me CHAN but that is because I am their boss. I really wouldn't call a 50 year old teacher CHAN nor would I ever use KUN in that way either. I also assume that you can call her CHAN too. Even if you are a girl of course this is all okay. If you are the same status in life and age or similar or you are above then of course KUN and CHAN can be used.
I have seen bosses refer to female staff with KUN also. So the rules for CHAN and KUN are not 100% solid. Just be careful using it with people you aren't familiar with since it can be rude if used at inapproriate times. Some people I have known for 10 years but I have never added CHAN or KUN to their name because they were higher in status than me or it just wouldn't be appropriate with this person.
So I would use CHAN with your friend since it is obviously a casual relationship but just keep an eye open for signs that she doesn't like it. I doubt this will happen though!