Who are his two friends?
'Rashi': Bildad and Tzufar.
What did they say that was improper?
'Rashi': You pained him with your words. His only sin was saying "Sam v'Rasha Hu Mechaleh", via the Satan "Im Shot Yamis Pis'om" (9:22-23). He spoke [improperly] due to harsh afflictions. You sinned via condemning him, and established him to be a Rasha - "Yir'asecha Kislasecha" (4:6). In the end, you were silent and defeated. You should have consoled him, like Elihu. Iyov had enough afflictions - you added to vex him!
Malbim: Their mouths spoke properly on behalf of Hashem, but this was unlike their hearts, like Iyov told them "k'Hasel be'Enosh Tehaselu Vo" (13:9). The primary speech to Hashem is what is in the heart, not the external speech of the mouth. In the heart, you did not speak properly like Iyov did.
Why did He call Iyov "Avdi", and say that he spoke properly?
Ramban: He confessed, and atoned for his improper words.
Malbim: His mouth spoke improperly, but his heart was complete with Hashem.
