Before 6 months, breastmilk/formula is nutritionally superior, better for their digestive system, and SO much easier. So I honestly don't understand why this is such a debated topic. It just doesn't make any sense.
And after 6mths, the UK guidelines recommend that EBF infants are started on vitamin supplementation because breast milk is inadequate. Does it suddenly become inadequate at 6mths? Or has it potentially been becoming increasingly inadequate over the previous few months and month 6 is the point where risks outweigh benefits and supplementation becomes advised?
Note that formula fed babies don't need supplementation. If we follow your logic, the best nutrition for babies is formula for the first 6 months of life and beyond. I'm not saying formula is bad (far from it, those that need to use it do so and credit to them), but there are many who question why formula feed when breast is better.
The point is, it doesn't make sense to you, but it does to another mother for her child. So if she's aware of all the guidelines and issues involved, and she chooses to start weaning before 6mths because that's what she feels is right for her child, who are you to write off her parenting choices?