"He could have done better."
Translation:Lui aveva potuto fare di meglio.
I think that's "he would have been able to do better". Presumably you know something that kept him from doing his best.
Well, I think that assumes that we don't yet know how well he really did. (We're waiting for the news.)
M: We should get the results soon. The race was over an hour ago. G: Don't get your hopes up; anything better than last place will be a victory for this guy. M: Hey! He could have done better than that.