I think this is a common thing to deal with so you wouldn't just be helping me by responding to this:
So there was this HB9 I mentioned in a tl;dr thread earlier that I had a pretty good first date with, we spent the same amount of money, passionate kissing off and on for hours and her being enthused to see me again at the end but no f close. Well when I went for a second date (the very activity she suggested we do, drive around and look at christmas lights) 2 days after the first date, 3 days later she responded:
HB9: "Hey! Sorry its taken me so long to get back to you. I didn't want to leave you hanging so i wanted to let you know that i was casually seeing someone else as well but this weekend we decided to get a little more serious and only date each other. Again, just didn't want you to think i was blowing you off cuz you seem like a cool guy and I'd be more than happy to be friends. If you aren't into doing that i understand

hope you're having a great week off!"
This was about half a day ago. I haven't responded. So here's how I see it. I wouldn't mind having her as a friend, she does have a fun personality, but I'm completely willing to blow my chance at being her friend for a chance to fuck her because I don't really need more platonic female friends. So with that in mind, should I sort of respond like, "sure we can hang out again sometime + [something funny]" and sort of tone down the flirtation until/unless we get one-on-one but not really acknowledge that she even mentioned she now has a boyfriend. She is still checking her online dating profile, so I'm not taking her commitment too seriously but do see this as being in 2nd place or worse right now, though he had a head start. Hope the guy's not you, haha.

What should I do? Is this worth more effort? Thanks in advance!