I don't think there'd be any harm in giving your pony some antibiotics; he likely sensed that your roo was on his last legs. I find horses are very sensitive to death and sickness and always try to boost the morale of the ill animal. But asking your vet would definitely be best so you don't dose him needlessly, which could breed resistance, which isn't good. By catching it this early, you should prevent the infection from ever starting.