I have a married friend who flirts. She sees nothing wrong with it, and says it doesn’t lead to anything. I believe it’s a sin. Is it?

It’s not a sin if the individuals involved are not married. It’s also not a sin if it is between people who are married to each other. But it is a sin if it is between a married person and someone other than her spouse, as in the case you mention. Your friend needs to get her act together. She owes such attention to her husband exclusively.