The Purple Dragon Knight/Banneret's Bulwark feature at 15th level states:
Beginning at 15th level, you can extend the benefit of your Indomitable feature to an ally. When you decide to use Indomitable to reroll an Intelligence, a Wisdom, or a Charisma saving throw and you aren’t incapacitated, you can choose one ally within 60 feet of you that also failed its saving throw against the same effect. If that creature can see or hear you, it can reroll its saving throw and must use the new roll.
(Sword's Coast Adventurer's Guide)
If my ally made a save one round ago can it still get a chance to reroll?