I can vouche for Willian in saying that he rarely gets pissed off, let alone make crap up. Unless you have any significant proof, it's just your word against his, and the story is in his favor at the moment.
Hell, he was talking to me on AIM when he got the party invititation, and I told him it might work out. No offense, but if a party invited me to go out somewhere and I'm expecting to get EXP, no, I don't want to kill/camp some NM unless it's in the way of getting more EXP. He has more than efficient means of making his own money on his own time - why would he agree to camp an NM with five seemingly random people (you can tell just by my signature that I don't have a job much higher than 60, so I'm not quite in the know of all H/NMs, so I'm assuming this would just be a money-gaining opportunity for him)?
How is anyone else supposed to know if you really saw the chat in /tells? You can't even prove that right now, let alone come up with any valid evidence that might prove Willian's story false. It's nice and all that you're trying to defend your linkshell mate, but it's probably better just to leave the talking to him and then possibly you could defend him further. I don't defend everyone in my linkshell for good reason (I'm not naming anyone), but I know good and well that Willian wouldn't just make something like this up. Perhaps there was some miscommunication between the party leader and Willian through /tells (someone zoned and it didn't quite go all the way through or something - that does happen), but it's a bit silly to expect Willian to actually be willing to participate in such an activity when it's more than likely that he was LFG to get EXP.
Also, might I point out this gem?
What are you so fussy over either way? He was under the impression that the camp was empty, so maybe, just maybe he was given the impression that you would be going there to get experience points. You didn't give him a definite answer, so you shouldn't even have expected him to give you a definite response. He went up there to EXP, your party decided to camp an NM instead, and Willian's only intention was to get EXP. I don't know about you, but if I was in that position, I'd just leave and find a party doing what I want to do. Just as Flameass said:
Maybe that isn't the most mature response, but I agree with it 100%.
Here, I'll change the situation a bit for you. You're Rank 5, and some linkshell is forming a level 57 party. There's five of them already, and all six of you happen to be on 5-1. You really don't give a rat's ass about your rank right now, you just want to level up so you can use your next piece of AF. The group invites you to party, and they tell you of some camp in Fei'Yin (I suppose you could fight weapons). You get there expecting to EXP, and they tell you that they know the camp isn't taken, but if it were, they'd do 5-1. You agree to go with them, and when they get there... suprise suprise - someone else is camping the weapons. Now, I know in this example that 5-1 is 50-capped, so it doesn't work out exactly the same, but I know I'd be rather pissed of some group took me to to EXP and then wanted to do a mission instead. Suppose they just told you to go in with whatever =<50-gear you have on (gimping yourself, I suppose)... the situation just now becomes exactly the same. If they don't like you leaving, oh fucking well. Tough luck, cry me a river, etc etc
