I would definitely NOT leave the female kids anywhere within reach of the billy.
My female kid came into season last year, although only late in November or December if I remember correctly. My girls go away on their "holidays" or have a "day trip", as we cannot keep billies at the moment.
I am not sure how far away they would have to be, but mine was VERY noisy and could be heard from the other end of the holding! Probably depends on the boy...