I'd say that 5 to 10 m is usually a perfectly safe distance to avoid a capsized boat.

When the capsized boat begins to right, then capsizes on the other side this is a sudden and unintended change, and I would take the view that it was not possible for the other boat to avoid, and it was not possible for either boat to avoid contact.

Added later:

If the boat that had been capsized was already upright and the possibility that it would re-capsize was obvious, I'd be looking at 10m rather than 5.

I agree with John Allan. The capsize on he other side cannot always be anticipated. It was not possible to avoid contact due to sudden unintended movement of the capsized boat.

Hard to make a call without seeing a diagram. Is this within a boatlength or two of the mark, or several lengths away?

If the latter, several lengths or more away, common sense dictates that you steer so that any point on your boat (including the end of your boom or spinnaker pole) is one mast length away from the capsized boat. IMO, it's not a question or 5 or 10 meters. It's whatever the mast height is. That will be less for an Opti but more for a J105. I'm saying this not as a judge but someone that's spent lots of time racing in windy conditions (San Francisco Bay).

If it's the former (right near the mark or in the zone), I'll leave it to those better informed about the rules than I.

Which tack was boat B on prior to the capsize?

Did she pass head to wind during the capsize?

If so, did she reach a close-hauled course on the other board?

When Boat B came up and then blew into/over onto Boat A it was on port although obviously still out of control.

Incident was at least 30m away from the mark .