[The Mouth of Sauron]

The Fellowship left Lorien to proceed down the Anduin by boat. The next day, Gwaihir brought Gandalf to Lorien, where Galadriel warned Gandalf that Boromir was coming under the unhealthy influence of the Ring.

Armed with that knowledge, why didn't Gandalf immediately send Gwaihir down river to warn Aragorn ? Come to think of it, why didn't Galadriel warn Aragorn even before the Company set off ? She certainly had plenty elven-muscle at her disposal to imprison Boromir.