who's responsibility is it?

[Andy]

As with most interference scenarios, this is a HTBT to determine whether or not we have interference.

I read the original post as F6 moving to field the ball and flinching or stopping due to what she believes is an imminent collision with the runner! That's interference in my book. It is the runners responsibility to avoid the fielder. Yes, its a judgement call and none of us (except the OP) were there. We can only go off of the picture we form in our mind from the description provided.

As far as whose call it is - Steve M gave a good guideline.