For instance, I sprayed this mint with my homemade vinegar weed killer, and a week later, while most of what you could see was dead, there was a wee bit of new growth right at the base. So I ...

She's come up with a "safe, cheap and easy DIY weed killer" that could be the answer ... Simply dissolve table salt into a gallon of white vinegar, then stir in some dish soap for good measure ...