My friend, when referring to his landlord, said: "what she could do with is 9 foot of rope"
I say it should be feet of rope, not foot. He says both can be correct: 'when referring to height or vertical suspension, it's foot, and when referring to length, it's feet'. For example, people say they are "5 foot 10". I think when it comes to people's height, it's a special case where it's okay, but in this specific context, regardless of whether you're referring to the rope horizontally or vertically, 'feet' is always correct and 'foot' is wrong.
Who is right here?