I go to a spinning class on Wednesday nights with a really nice and fun friend. I get a little embarrassed because she flirts endlessly with the instructor. Maybe it's just not my business and I should let it go. Well, she flirts with all the men we know actually. Should I say something or let her be?
Every friend is a package. Perhaps one of the reasons why your friend is "fun" is because she is so friendly and outgoing. Her flirting with the spinning instructor may be innocent but you haven't mentioned whether she's married, whether he is, or whether her flirting is innocent.
If you think her ways are going to get her in trouble or are embarrassing her, you probably should say something. Another reason to say something is if your embarrassment is getting in the way of your friendship.