Your buddy followed through on his promise to retire. You were right in your assessment of his principled character (in this situation, at least).
No, he did not "keep his promise." His promise was not only not to run again, but to resign immediately and let a new Senator from Idaho take his place, if his challenge to his consent conviction was denied. It was denied; he did not resign then.
Craig thought that the Senate Ethics Committee would be too timid and toothless to throw him out. Unfortunately, he was right. The Committee issued only a slap on the wrist. From what I see of Craig now, the only reason he did not file for reelection is the correct assessment that Idaho voters, unlike Massachusetts voters, will not tolerate reprehensible behavior in their Senators.
Otherwise, Craig's ego would have led him to file to run again.
Congressman Billybob