I have been in this exact situation.. I blew her off... I debated about what to do, evenutally, I thought the best option was to go to her about it. She went nuts!! I told her to back off, and she just started yelling, I told her not to be a bitch about it, and to chill, I was just trying to be the best thing for them... When my BEST FRIEND found out about this, he was pissed at me for calling her a bitch, and avoided the issue. I would tell you to tell him but play it down... tell him you felt like she is coming on a little strong, be prepared for him to get defensive.....
but its better to a be a friend and him be mad then let it slide and she fucks someone else... gl.
it sucks