I don't know maybe as another redhead myself (not that we all act the same lol) I can attempt to figure out what she's doing based on possible reasons I would be acting like this with a friend (Disclosure: I am not a perfect person, and although I don't go around trying to hurt others, sometimes I screw up). I will say, definitely sounds like she sending out some kind of signals. I do stuff like this, especially if I’m in a position when I can’t make a move on the guy myself. Like I’m too chicken shit, or I’m already attached.

If I was doing this, the most likely reason would be it was be because I was attracted to my friend...maybe all along...maybe I've grown into it as the friendship has grown. Maybe she doesn't think you feel the same. Maybe she's been throwing out subtle signals for years and now she's ramping it up to make sure she gets your attention. Maybe she truly loved the man she married and now has realized that she isn't as happy or in love as she thought she would be. Maybe she wants to have a really hot affair with someone she can trust and who she loves.

Sorry, a lot of maybes there. I guess you really have to ask yourself what you want. You say you don't want to break them up, but would want a relationship if she left her husband? Would you want to participate in an affair with her? Would you be ok with that? (As an aside do you know for a fact that they don't have an open relationship?). And despite what other people will say, just because she's doing this with you doesn't necessarily mean she'd do this to you if you were in a relationship. You've been friends for a really long time, longer it seems than her romantic relationships. This could be an exception. It was for me when this happened with my current SO (who was my friend when I was still with my ex). Almost 11 years together now.

I would try to ask her, unless you are enjoying your flirtation. It does sound tortuously fun (that might be the redhead in me talking), but eventually there’s a good chance the both of you will crack and just end up fucking. And then you’ll have to figure everything out after the fact, with all the added complications.

So try and talk to her. Make it casual though, like “if I didn’t know any better I’d swear you were flirting with me”. Just talk to her, she’s your friend after all.