Friend is ****ing over two girls at once

One of my housemates is seeing a girl (call her Girl 1) at the moment. They're not strictly boyfriend and girlfriend, but he's definitely pushing towards that situation. On nights out, whilst Girl 1 isn't there, he ends up getting with and going home with another girl (Girl 2). Girl 1 has caught him doing it before - and was really angry at him - but he somehow managed to worm his way out of it and convince her to keep seeing him. In fact, we went out on Friday with some friends, and I left early - when I woke up, he wasn't in the house. He came back a bit later and told me he'd stayed at one of our mates' houses (which seemed perfectly plausible). Girl 1 actually came over that day to see him, and I heard him telling her the same story. Turns out, as I found out today, he'd actually drunkenly called Girl 2 up and invited himself over to her house - not sure about other details but it's not hard to imagine what probably happened there. He's sneakily done this a few times. Once, he went to a friend's house for a couple of drinks and didn't come back 'til the next day. He told us, his housemates, that he'd stayed at that friend's house, and he'd told the friend in question that he'd come home and gone to bed. Obviously it only took a short conversation with said friend to work out that he'd done neither, and had actually gone off to Girl 2's house. What gets me more than anything is that he's even lying to his own friends about it (even when we tell him we know what's going on [in some cases even Girl 2 has told us he's been at hers, and he still lies about it!], he denies it and denies it until we stop talking about it), which blatantly shows that he's up to no good. Even worse, he saw Girl 1 and another girl at the same time for a couple of months last year - Girl 1 found out but the other didn't, so he just ditched Girl 1 and went out with the other until he got bored (and he cheated on her a couple of times then as well). I know he's technically only seeing Girl 1 (although he's been seeing her for a good four/five months now), and that if he just had the occasional accidental drunken fling with a randomer it'd probably be excusable, but he's consistently seeing Girl 2 as well. My dilemma is this: he's otherwise one of my best friends, even if he is a ****ing idiot. Girl 1 is a good friend of mine as well, and I want to make her aware of the situation before she gets into something she might regret later. However, it's not really in my place to tell her that he's been lying to her, and I know he'd never forgive me for it (which, yeah, is his own problem, but I don't particularly want to have to go through the stress of losing a friend over this). I've never liked the way he treats girls, and I've always wanted to cut in and explain everything to the people he's lying to, but again it's kinda not my business. Sorry about the long post, but I'm stressing out horribly over this. What the **** do?!

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