I live in a house full of 7 college-age people, all friends I have known for anywhere between two to nine years... except for our newest household member Jay who we all just recently met.

Now, my friend Dianne (who lives in the house as well) Shares a room with her boyfriend Tim. Ever since Jay moved in, Dianne has built a little obsession with him, and what started as innocent flirting has graduated to overly-affectionate cuddling, hugging, and touch-gestures (arms around each others waists etc.). The rest of the household is alarmed, because we are friends of both Dianne and Tim (Tim actually being my ex-boyfriend, and still a very good friend of mine) and Dianne's actions toward Jay are getting reckless. She insists to everyone that she and Jay are just becoming fast-friends, and she means no harm with any of her actions. Tim, being a little bit of a pushover is obviously scared to refuse giving her what she wants, so he has so far stayed silent, though we can all tell it's driving him crazy. He witnesses most of what they're doing, and they half-heartedly they include him as if pretending 'Hey, we're all friends here', but the rest of us see past their antics.
She's posting pictures her and Jay on facebook, taking off work so she can stay home with Jay while Tim goes to work... and each day they're getting more and more 'comfortable' with each other. I'm afraid of letting this go on for too much longer, or someone is going to get hurt, people are going to take sides, and drama will consume the otherwise harmonious household.

How do I talk to her without spooking her away? She's hinted she's curious (after all, Tim is the first and only guy she's slept with) and I worry what she and Jay are already doing behind closed doors. The rest of us are anxious, and we're waiting for Tim to snap, or for her to lose control. I'm afraid that if I'm blunt, she'll feel offended and refuse to let me in and talk to me. Is there another (gentler) way I should go about explaining why what she is doing is wrong?