σπεῖό … ἕσπεο
Within the epic tradition this verb can have a marked meaning, signifying a god who accompanies a hero and gives him special protection or help. We see the same verb later indicating that Athena is indeed accompanying (ἕπουσαν, 10.516) Diomedes—Apollo is said to see her doing so. Athena has already signaled her presence through a bird sign (10.274–275), and will later speak to Diomedes (10.507–512), although she remains unseen throughout. In other cases, Athena “accompanies” mortals in disguise: at Odyssey 2.287 while disguised as Mentor she offers to accompany Telemakhos on his journey, and at Odyssey 6.32 appearing to Nausikaa in a dream in the guise of the daughter of Dymas she says she will accompany her. (Although she does not go with Nausikaa in this form, she is present at the washing place.)
Parallel to Athena accompanying Diomedes and Odysseus on this night mission, Iris tells Priam in a dream that Hermes will accompany him on his night mission to the Achaean camp to ransom Hektor’s body (Iliad 24.182), and of course, Hermes does so in disguise (see above on 10.1ff. for more on how Priam’s mission is similar to the ambush theme). Thus we find that this particular use of the verb for gods accompanying mortals has associations with dreams, disguise, and night missions. As we have noted in “The Poetics of Ambush,” there is thematic overlap between such missions and journeys. Therefore, in addition to Athena saying she will accompany Telemakhos on his journey at Odyssey 2.287, we can also include among the examples of this theme Nestor’s proclamation that the gods in general accompany Telemakhos on his journey: τοι νέῳ ὧδε θεοὶ πομπῆες ἕπονται (Odyssey 3.376). Athena has a special relationship with Odysseus (see the commentary on 10.275 for more on that relationship), and that intimacy extends to his son. Here, Diomedes asks for the same kind of consideration based on her relationship with his father. We know that Diomedes indeed has such a relationship with Athena from Iliad 5. See also 10.291.
Finally, we may compare this use of ἕπομαι for god and mortal pairs to its use for teams of warriors; it is used not only of the team of ambushers here in Iliad 10 (10.222, 10.227, 10.246), but also of warriors who team up in daytime battle, as at Iliad 11.472, 15.559, and 16.632. In each of those latter cases, the one warrior asks the other to join him in pushing the enemy back, and when the second follows/accompanies him, he is called “a man equal to a god,” using the same formula: ὣς εἰπὼν ὃ μὲν ἦρχ’, ὃ δ’ ἅμ’ ἕσπετο ἰσόθεος φώς (“So speaking he led the way and the other man, equal to a god, accompanied him”).