60 Sec,
You really don't need anything to support your winnings. As I said, the IRS is happy to take their cut.
If I were your friend, I would report it. You are right, it sucks either way. You can take your chances and not report it, but if you are making a significant amount then you could be looking at jail time if you get caught.
Or, you could report it, and then one third of your hard earned money will go to government. Either way, it does really suck.
If you are a pro gambler living in Vegas, then there are a lot of expenses that you can write off, so the tax hit is not that bad.
In this case, you would have to declare yourself as a professional gambler.
Being that I am pretty sure that your friend is not a professional gambler, he is not going to be able to get all of these additional write-offs.
Just tell him to report it, and he will sleep better at night.
SBBC,
You report your winnings, and then offset the losses against them.