Despite all the sensible and helpful suggestions above. I get the feeling that your friend does not want solutions to a problem she does not want to recognise and while you bear the brunt without complaining she will continue as she is and you, it is clear, also do not want to tell her because she can be difficult (I have a friend like this, fortunately I do not see her often)
You say that you too have been ill, so I suggest you tell your friend, with great sadness and concern, that your doctor has read the riot act to you over your continued health and laid down rules for you to help you stay well. These mean that you have to avoid all physical and muscular stress, which means you will not be able to give her support when she is walking or carry her handbag. To support your argument, do as Smileless suggests and turn up with a walking stick.
This then puts the ball in your friend's court. Either she makes adjustments; gets a parking ticket, uses a walking aid or whatever or she has to decide to cancel or reorganise your meetings.
If she tries to get you to continue to help her, just say, at the point where you have had to park a long way from the venue and she hands you her bag, 'Sorry but I cannot risk my health by continuing as before, the doctor has been quite specific.'
As for paying, when it is her turn to pay, just remind her as she leaves the house to get into the car that she is paying and is her purse in an accessible location.
It will be a bit of a shock at first. I suspect you have let your friend rule the roost because you like her despite her sometimes overbearing ways (my friend is the same), but their are times when you have to dig your toes in.