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.
If your spinning friend is consistently flirting with "all the men you know", it suggests that she might be very insecure and as a result, needs continual affirmation of her attractiveness from men to prop herself up. It could be that she has no insight into the way she is coming across and may benefit from some gentle feedback from a good friend. Depending on the nature of her flirtiness and your relationship -- and whether it's gotten her into trouble -- you may want to encourage her to talk about this issue with a mental health professional.