I had a friend like this when I was young. She always managed to go out without any money or had forgotten her purse, etc. etc. Met up with her 20 years on and she was exactly t he same, and she was better off than me.
There are people around like this and it has nothing to do with being hard up. Unfortunately, they often lose their friends.
I don't worry a bit about who pays for what, but I don't like being taken for a mug, and this is what your friend seems to be doing.
Next time you arrange to go out, phone and ask her if you can use her car (just before you go) because you have a problem. Tell her you are taking a picnic and when you get to the car parks don't have any change on you. Carry on like this a few times and I'm willing to bet her friendship won't be so strong.
If you are genuinely hard up then you can't expect to rely on your friends generosity - she's a sponger.