Foul or no-call?

[Lotto]

A friend of mine posed this situation to me. I know what I'd call, but I couldn't articulate the reasons very well. So let me turn it over to you and see if you can do a better job than me.

A1 is on the floor, on her back, has the ball. B1 dives on top of A1 and ties up the ball. You judge that B1 was just trying to get the ball. A1 is not displaced when B1 lands on her, but B1 lands solidly on top of her.

Foul on B1 or held ball?