Aragorn is concerned that Saruman may have further tricks up his sleeve.
'Saruman seems to be in a tough position, but I would counsel caution in our dealings with him. I do not doubt his ability to strike even now, and a cornered beast can still do great harm.
'Gandalf went ahead to the base of Orthanc. Find him and see what he thinks of the situation.'
Gandalf is at the base of Orthanc, considering what to do about the Wizard waiting within.
Treebeard is by the base of Orthanc.
Gandalf has asked you to speak with Treebeard while he considers what to do about Saruman.
Treebeard believes you should search within the Ring of Isengard for any sign of your captive friend.
Gandalf is at the base of Orthanc, waiting to hear what you discovered in your search of the flooded Ring.