In classic Mumsnet style, Did you mean to be so rude?
This is part of the home OP and her husband bought. An allotment style plot for her DH to benefit from given his health issues, and an asset with value. Just because the previous owner let a pal use if when she had no use for it doesn't mean Barb can be petty or obstreperous about it.
It isn't hers. It never was. She had the extraordinary good fortune to enjoy a free allotment for some years. Her 'luck' has morphed into 'entitlement' in her mind, and she's bang out of order for that.
lIf Barb had any class whatsoever, she'd clear her stuff out and pass on any knowledge about the soil or recent crop rotation to the new owners like a sane person would.