Hello, I'd like to help.This is a really sticky situation. And it comes down to how far you are willing to go to get along with a neighbor.
Fox feces in general does tend to have a more pungent odor as their anal glands (scent glands found in all canids on either side of the rectum that release when stool is passed) produce a more "musky" odor. The odor produced can be influenced by diet as well though.That said stool is also a mirror of diet. Most foxes also eat whole prey like birds and mice, moles etc so will often have a fair amount of hair and/or bones in their stool. But fox that live close to humans will eat cat food, bird or dog food left out for pets as well as garbage or restaurant leftovers so their stool can mimic dog stool.
You could set up motion detector cameras to "catch her dogs in the act" and prove that these stools are from her dogs.But in the interest of keeping the peace you might just decide to clean up after her dogs. I know it isn't fair or fun to do. And most likely you are correct and this isn't fox stool. But being right may not be as important as being on good terms with your neighbor.