As I mentioned in a previous post, Telemachus was when first seen sitting and contemplating what to do about the suitors and his family’s situation more generally. When he sees Athena/Mentes coming, he gets up and greets her/him and invites her/him as a stranger to the main meal of the day, lunch.
One should note that the Greek greeting χαῖρε (used even today) is literally the command to “Rejoice!” or “Be happy!” in classical Greek. The word is often used in both the sense of the greeting and its more literal sense, with word-play being common. (Anyone reading a translation may find this treated bizarrely by the translator in many cases, to the great confusion of readers.)
One should not be surprised that Telemachus addresses Athena/Mentes as “stranger”, even though Mentes is a friend of Odysseus. Most literally, if Mentes ever came to Ithaca previously, the visit would presumably have been when Telemachus was a small child. Of course, just because Telemachus address her/him as “stranger” does not necessarily actually mean Telemachus does not recognize the person of Mentes; Telemachus is after all his father’s son– not to mention the equal influence of his mother Penelope.
More importantly, by addressing Athena/Mentes as ξεῖνε, Telemachus is explicitly offering Athena/Mentes the status of a guest-friend. The custom of guest-friendship pervades the Odyssey. The suitors who abuse the status are committing an offense against Zeus. Telemachus’ actions will stand in direct contrast as he gives Athena/Mentes all the rights and privileges due.