In this case I'd probably dice for which downed combatant the berserker continued to attack, rather than assuming it was against the fighter, just to provide some impartiality to the rage.
But, the berserker rage has always seemed a bit odd to me: 7/8ths of the time they continue attacking *enemies* (alive or dead) for a # of rounds equal to CN, but 1/8th of the time they *continue* for 1d6 rounds after *enemies* are vanquished. So does this mean, for example, that a berserker of 15 CN could continue attacking for 15 + 1d6 rounds 1/8th of the time, with the 1d6 rounds being against any living creature? Or would they be exhausted for those 1d6 rounds (occurring as they do after the 15 rounds), and so not in an "uncontrolled" rage? Apart from its D&D legacy, is there historical precedent for berserkers to continue attacking any and all beings (friend or foe)?