I've had the pleasure of photography three dancers over 2008. They have become friends so I was enlightened to their business and the dollars they earn. Small clubs a girl will make $1,200 to $2,000 in a week - not including tips. One dancer told me that she made about $20,000 over a two week period (tips included).
So I don't believe they can't afford it. These three gals just wanted shoots for themselves so I charged them only $200.00 each. But if it was for promotional material, then the rate would be $500.00.
Your problem is that you are feeling guilty because this is a friend asking you. I've used friends that were plumbers, electricians, and mechanics. Never was the work "free" - maybe a small "friend" discount.
You'll be treated professionally only if you see your work as being professional. And then it has value to it. Don't be afraid to ask a fair price. What's the worst that can happen? Someone else takes on the responsibility and you still have to pay to go into the club.
Let us know how this ended - with a few phtos maybe (if they end up paying you).
“You don't take a photograph, you make it.” - Ansel Adams