The bard spell Saving Finale lets the bard end his performance to let one of the performance's targets reroll a failed save:
You must have a bardic performance in effect to cast this spell. With a flourish, you can immediately end your bardic performance when a creature within range affected by your bardic performance fails a Saving Throw, allowing the subject to immediately reroll the failed Saving Throw.
The question is, how does the bard know that the creature that he wants to help has failed a save? Phantasmal Killer reads:
You create a phantasmal image of the most fearsome creature imaginable to the subject simply by forming the fears of the subject’s subconscious mind into something that its conscious mind can visualize: this most horrible beast. Only the spell’s subject can see the phantasmal killer. You see only a vague shape. The target first gets a Will save to recognize the image as unreal. If that save fails, the phantasm touches the subject, and the subject must succeed on a Fortitude save or die from fear. Even if the Fortitude save is successful, the subject takes 3d6 points of damage.
If the subject of a phantasmal killer attack succeeds in disbelieving and possesses telepathy or is wearing a helm of telepathy, the beast can be turned upon you. You must then disbelieve it or become subject to its deadly fear attack.
Since only the subject can see the phantasmal killer, if the bard fails his Spellcraft roll to identify what the enemy is casting, does he miss the chance to save his ally?