Do anything you can to ensure that she DOESN'T go. Even if she is "good," what do you think will happen when you all get back and she starts flapping her gums to all of the other guys gfs about what went on? I think it's pretty safe to say that this probably won't be some kind of Disneyfied cola-and-sightseeing family vacation type trip, and things will probably go on that would get any of your buddies put in the doghouse with their SOs if they were to find out about even 10% of it. And this girl would love to be the one to tell them all about it. Her just being there would change the entire mood, and kill the whole trip.
Case in point, I went on spring break in Daytona beach a couple years ago, and one guy brought his GF. She proceeded to pass judgment on our drinking, wouldn't shut up the whole time, chased some hot girls in thongs out of our room, calling them "sluts" :| (OOh, that one pissed me off) and went to bed every night at about midnight, or 1. And then had the nerve to bitch at us when we came in later and woke her up. We were at a bar that had a wet T-shirt contest going on, and she got all pissed off that we were watching it, and loudly called the girls "sluts". She even made her BF leave while it was going on and didn't allow him to watch it. Everyone wanted to punch her in the face after the first night. Of course, our buddy had to take her side in all the arguments that ensued. It was painful watching her emasculate him constantly. His degree of whippedness boggled the mind. In short, her presence TOTALLY ruined our trip.
He can't come up with the cash, he shouldn't go. Period. Besides, Vegas is no fun with no money anyways. DO NOT take her along, under ANY CIRCUMSTANCES. Please.