Why didn't Gandalf or Galadriel warn Aragorn about Boromir ?

[Kitanna]

Would Gwaihir have even bothered doing this for Gandalf? Certainly he owed the wizard, but what did he owe to Aragorn to deliver a warning?

Originally Posted by The Mouth of Sauron

Galadriel warned Gandalf that Boromir was coming under the unhealthy influence of the Ring.

Unhealthy influence doesn't mean there is a guarantee of action (as others have said). Granted the race of man had failed in the past to withstand the Ring's sway over them, but that didn't mean Boromir would fall. Wise as Galadriel is she didn't have the choice to detain Boromir. Lindale points out keeping Boromir would have destroyed the balance between men and elves. As it was the men in Rohan and Gondor feared the "witch" who lived in the Golden Wood.

What good would keeping Boromir have done? Likewise, what good would warning the Fellowship have done? What could they have done, leave Boromir in the wild to keep him away from Frodo? Things perhaps could have gone smoother for the Fellowship had they been able to see all ends, but there's a destiny set before each one of them it seems and a warning would have upset that.