I guess "know" is more like I'm 98% certain nothing would happen and the other 2% is "I really don't want to think anything would happen."
Being as she and I aren't really CLOSE friends, I can't say for sure that I know she wouldn't get it on with a friend's significant other. And being as I can't get inside his head, I can't say for sure that his conscience could take over his lack of inhibitions if he were smashed and dealing with the ego trip/hormone inundation that would occur if someone he found attractive propositioned him. I have no idea if they were having any alcohol last night or not. He didn't seem off when I talked to him over instant messenger so I assume not, or at least he wasn't drunk (yet?).
Usually I can keep insecurity and jealousy curbed, but I have never thought there's a reason why someone in a serious relationship needs to spend the night alone with someone who's not their significant other; if someone else had posted this and not me, I likely would've been appalled by it and advised them certainly not to treat it so gently as I've been doing.
Part of my worries also are that I was talking to him online last night and he said something about how they were going to watch a movie or TV or something so he was going to go, and if he didn't come back later that meant he'd "crashed" (i.e. stayed up too late, gotten really tired, and fallen asleep). At first I thought "okay, big deal, you're going to get tired and go to sleep without talking to me more, it's happened before." But once I thought about it, I thought, "WAIT. 'Crash' as in fall asleep where you're sitting? Does this mean the plans are not that you're sleeping in your room and she's sleeping in the living room but you'll both be in the living room????? The SMALL living room where two people sleeping on the floor can't be that far apart?" Perhaps that is not what happened, but I can see no other reason that if he actually got up from the couch or wherever and went to his room to bed, that he would not first go the two feet to the computer, and sign off the instant message program before he went to bed.
I just really hate this. I mean, I've overlooked the flirting, because that's what she does, and I'm not going to grudge him the ego boost of someone besides me hitting on him because it probably does him good--I even didn't say anything the time last weekend when our whole group was together (this time we were at another friend's house all with sleeping bags on her living room floor) and he was lying there with her sitting next to him running her fingers through his hair (I swear she was, I had to keep myself from saying "OH F*CK NO, I KNOW MY EYES ARE DECEIVING ME AND YOU'RE NOT STROKING MY SO'S HAIR!"). But this, is starting to be a little too much...