I do have one female friend who believes that men feel good about playing that role. Even though she has plenty of money she would think it would somehow emasculate a bloke to suggest that she pay, she usually makes a token gesture but never would she think to suggest that she pay for petrol or even round for round.
When she drinks in female company she would never consider that her very expensive drink versus a pint was unfair. She is one of the nicest people and if you needed £2-3 hundred quid she would give it to you..
There is no logic to some people.
I sincerely hope your "friend " falls into this category and simply doesn't actually believe that you might be skint. In her mind £50 for a two day visit is about fair.
Perhaps next time you get any surprise visitor you agree that you'd love to see them BUT suggest up front, that you'll do the running round ( fuel costs ) if they pays for everything as you are strapped for cash, explain that you can use use credit card for fuel. very plausible. You simmply look the other way when it comes to the till or the bar, have the confidence to look them in the eye and smile because it is fair you are putting them up and you cannot afford to subsidise.
Good luck and I hope you can enjoy your next visitor.
On the issue of animals for research "The question is not, 'Can they reason?' nor, 'Can they talk?' but rather, 'Can they suffer?'" Jeremy Bentham