Picture posting is enabled for all :)
Grow it on and see; I've never seen a knotweed seedling. I'm sure I've read somewhere that all the knotweed in the UK belongs to a single clone, and I'm not sure that it produces viable seed, though I'm not sure why it wouldn't.
It can't go anywhere while it's in a pot. Get it grow, and if it turns out to be knotweed, kill it. You could always chuck it in a pan of boiling water to be certain. Or put it somewhere to dessicate for a few months.
Thanks everyone - she is really worried as she planted hers out yesterday - this is one she gave us!