I don’t remember the source but the antidote to hate is to do favors and be extra nice to the person in question. So that’s what you should tell the guy, since he apparently asked for your advice. For example, if shayach, offer him a car ride, inquire about his family, compliment his Shiur or find other ways to be nice to him.
If there’s truly no bad will between them (where asking/ granting forgiveness may be a prerequisite) the hate should subside and hopefully ahava will follow.
Another thing to bear in mind is that flawed character traits may bother us more if they are similar to our own. We don’t want to face our demons.