I feel you on this one. We have a close group of 3 other couples whom we spend quite a bit of time with. For the most part, it's not an issue - we like to get together for board games, and spend a lot of time at the park together in the summer.
However, one of the couples - or more so, the female half of one of the couples - does her best to get us all spending money, and it's driving me batty! She's the only one in the entire group who likes to go out to the bars (she's also the youngest, which may have something to do with it), and just recently she decided she wanted to spend her birthday "in the city", and organized an overnight trip, involving recreational shopping, a ticketed event, dinner out, plenty of drinks and a hotel. Now, I know we could have just not gone. In fact, I turn her down frequently for this type of thing. We did go for this trip, and tried to keep our expenses as low as possible (we carpooled with another couple, got a deeply discounted hotel room, didn't drink alcohol and kept miscellaneous spending to a minimum) and that's on me.
But it bothers me that she thinks this sort of thing is necessary on a regular basis, and she gets quite offended when she's turned down. I've tried to suggest alternate activities as much as possible, which helps a little but quite frankly if it wasn't for the rest of the "group", I wouldn't be friends with her at all.
And, of course, this is the couple on one income, with a fancy new SUV, who just bought the most house they could get financed with 5% down. They DO NOT HAVE the money to do this shit. I've been getting out of stuff lately by saying we're saving for a down payment of our own (which is true, but I insist on having 20%). But I really don't know what the long term solution for this is.