How to get rid of certain so called friends? they need me only when they require help and shit

My Heart goes out to you, I suffer from this friendship type too. I have an older friend over our road. she has been off her wheels just now recovering from minor surgery, she has a son who can easily drive over and help her with shopping etc. only he rarely does. I like her welll enough but her favourite conversation topic to me is guilt trips. example I WISH YOU AD INVITED ME TOO WITH YOUR FRIENDS, I AM REALLY FED UP ALL DAY ALONE, MY SON IS SO WORRIED ABOUT ME , HE PHONES ME DOZENS OF TIMES , MY BLOOD PRESSURE IS THROUGH THE ROOF AS MY FEET ARE BAD I CANNOT MANAGE TO GET OUT I IF ONLY YOU WERE THERE....MY OLDER FRIEND CAME TO MY SIDE AT THE GRAVESIDE OF ANOTHER FRIEND`S FUNERAL WHEN YOU HAD TO LEAVE TO GO HOMETO WALK YOUR LAME DOG.
I am at my wits end dreaming up a good sincere punchline to let this needy friend know I can do only so much for her. Hope you find the answer to your problem very soon good luck