To be as fair as I can be to my friend, he was more into grass keeping once upon! But he has firmly asserted, in most recent discussion, "I have seen the pink flowers myself". So perhaps (as you say Scarlet.D) there is a mix of the weeds - but I really doubt that.
Must go search "pink-flowered knotweed"!
Whatever, it has been agreed that he will leave a small 'stand' of weed to reach early flowering stage for re-evaluation/final identification later this year. I can't wait (although I suspect the real culprit will be identifiable a bit before then). We shall see.