Omfg. I just fucking explained this shit.
BOA USED SLAVE ARROW FIRST. AT THE SAME TIME, BB WAS FIGHTING SERAPH HAWK. AND COBY WAS NEAR THAT FIGHT. THEN, AFTER BOA TURNED VASCO AND OTHERS TO STONE, BB WAS DONE WITH THE SERAPH AND WENT TO BOA AND CAPTURED HER USING YAMI
THAT'S WHEN COBY ALSO HOT THERE.
BB LITERALLY SAYS THAT EVEN HE WOULD GET TURNED TO STONE IF HE LET BOA GO. SO, IF COBY GOT HIT BY A SLAVE ARROW, HE WOULD BE STONE. THUS, HE DIDN'T GET HIT BY SLAVE ARROW. BECAUSE HE WAS FUCKING NOT THERE.