wow, sounds like an interesting afternoon. it also sounds like owner A is being pretty cool about the whole thing, and since nobody was hurt, you have a pretty good war story for the clubhouse/beach. now for my opinion.

boat A tacked. according to rule 13, After a boat passes head to wind, she shall keep clear of other boats until she is on a close-hauled course. During that time rules 10, 11 and 12 do not apply.

regardless of whether A had rights or not before tacking, she would have had them after tacking (she was starboard after the tack), but:

either:
according to rule 15, When a boat acquires right of way, she shall initially give the other boat room to keep clear, unless she acquires right of way because of the other boat’s actions.

or:
according to rule 16.1, When a right-of-way boat changes course, she shall give the other boat room to keep clear.

either way, i read it as boat A is at fault.