A couple of months ago at a party, I saw my friend flirting with a handsome older guy. At another party a couple of weeks ago, I met him and he asked me out. The guy knows that she and I are friends, but she doesn't know that I went on a date with him. How can I casually mention that I am going to see him again and ensure that she's O.K. with it without making this into a big deal? I should mention that she flirts a lot, as do I. I don't mean that in a bad way, just to say that I recognize that her flirting doesn't necessarily mean anything.
Flirting With a Friendship Disaster
If you don't want to make it into a big deal, I suggest not making it into a big deal! Asking your friend's permission invites her to find a problem with your date, when it's really not her business. If you know that she has a huge crush on him or that she asked him out and he said no, that's a different story. But based on your description, she flirted, you flirted, he flirted, you went out. No big deal indeed.
Since she's your friend, your dating life is probably a topic of discussion anyway, so I would approach your conversation about this guy the same way you would talk with her about anyone else, with one small exception. Instead of saying, "this new guy I met on JDate," or, "that guy from the happy hour," say, "I'm going out with Jake. I think you met him at Rachel's party last month." You don't need to describe him as, "that guy you flirted with." It's ultimately an unnecessary detail.
There is, of course, the possibility that she'll be resentful, but this is a time to ask forgiveness, not permission. If she seems upset when you tell her, then you can open up the conversation and ask if there's anything you should know about him. Maybe it's that she likes him. Maybe it's that he's dated everyone who was at the party that night. Maybe it's that he's a great guy who flirts all the time but rarely asks people out, so she's a little jealous. Regardless, one night of flirting doesn't amount to much in the grand scheme of the single life. So you can relax and enjoy your second date and, perhaps, beyond.
If you're still uneasy, consider trying to find out from the guy directly or from another mutual friend if anything else happened with your girlfriend beyond what you saw at the party. For future reference, if you're considering dating a friend's ex (or, for that matter, her roommate or brother), getting permission, or at least informing her before she hears it from someone else, is certainly appropriate. In this case, all's fair in parties and flirting.