in a bit of an awkward situation, any advice?

so i have a good friend who is dating this girl whom i know as well they're both cool people. We for the most part all hang out in the same group of friends throughout the week, grab drinks, food etc. For some reason she flirts with me a lot and she's pretty good looking as well. Now here's my gripe. If she likes me and is attracted to me over him then why the hell did she start dating him??? i mean he's a good friend and im not trying to step on any toes, i actually won't act on it or make a move because i respect friends and that's not my nature to intervene. So whenever I see her she gives me real big hugs like she hasn't seen me in ages and always says how much better her day is when im around. I appreciate it but before she was with him why didn't she say all of that then? completely confusing. I also don't want him to feel im moving in on her or anything which im not at all i just hang out and she follows me everywhere. Any advice? should i tell him something or just try to avoid her flirting with me?