Finally, I see the story and how this story evolve into such a lame scene and let me tell you this frankly - this whole damn thing is because of a girl, who is not only immature, but rather foolish. Let me do an analysis for you and I am sorry, but I feel that the heat of furiousness is brewing -
I will be frank and firm.
'Dan and Joyce have sex and was never gf/bf' - this statement already spoke much of Joyce's character and I wouldn't elaborate any further.
'Then they invite Joyce over to Greg's house. When Dan's sleeping, Greg and Joyce fell in love or something. Then they did something close to sex. ' - This statement further confirm my above comment. You said Joyce was so aborable everyone sees her likes her, and I don't understand why she would be so easy with herself. Well, in the end, its all boils down your own childhood psychology and karmic relations.
'The next few days Greg and Joyce have been meeting each otherÂ’s and they had sex. Greg wants to tell Dan about it but did not know how.' - from here, a third confirmation of Joyce easy nature and one new point: Greg is such poor friends. Seriously speaking, would anyone fark their own good friend's (13 years eh?) potential gf, crush, or gf? If so, Dan needs to look at him from other angles. I don't believe in years of friendship = better as it ages. I believe in how the friend conduct himself with him.

The reason why Greg would claim that Joyce bj him is because of men ego. I was in a guy school for 10 years and understand every single bit of that part. All part of guys talk.

Alan and Dan's daddy is such an extra. Seriously speaking - what the hell does it got to do with them. People always try to get involve in one another's relationship problem and ended up laming this whole issue.

And I don't believe this: After so much happening, all she was crying about is that Gerg and Dan's friendship is falling because of her and she felt that she was a b|tch because of that. OH MY GOD! Didn't she realise that the ROOT of the problem was because she was so farking easy with herself? If I was her, I would slap the hell out of Gerg and destory all contacts with him. Its ok when one is easy with her body (if she was the willing party) - but integrity, one has to protect.
'Dan's dad ask Joyce to go to their house for drinking' - obviously, even in my preception something was wrong and his dad's character & motive leave much room for questioning. As a friend, how can you even leave her in a lurch and left for tution? You must bring her out too! Isn't she your friend too?

This scenario - has come so complicated is of lust. She(4) zhi(4) tou(2) shang(4) yi(4) ba(3) dao(1). (Direct translation: Lust is like knife hanging above head.)
Advice?

She is the source and root of all problem: She cannot be use to solve this crisis. She needs to avoid all these people. Before avoiding - she must either write a letter explaining everything to Dan and Gerg about her decision OR meet them (IN PUBLIC PLACES AND NOT IN EACH OTHER HOUSE FOR GOD SAKE - Oh no, I am screaming because she is so naive.) and spoke to them (not together, but as individuals)

You as a friend, you have to watch over her and MAKE sure she don't go over to their houses and especially any form of meeting for Dan's daddy. And do you know that all these happening is sufficient for you to dial 999 and scream rape at the Dan's dad face if our preception of his motive is true? Of course, she didn't say a word about it and therefore no point to dial either.

She hope that the both of them can patch back their friendship? Can... but she must disappear because her appearance will bound to trigger memories and resentment. Take action as soon as possible - use all woman's tactics of avoiding - I am sure she will know how.

P.S: The profit from a girl is often an expense of a friendship.... - CloUdiSm -
Cheers