"I love you for her" is a remark that someone makes when he likes someone because of some third person. This might happen when the object of one's interest is not within our reach and we try to reach them via their friends or some other acquaintances. At times, desperate guys go ahead and hit on the best friends of the girl who has acquired their fantasy. This is solely done to get close to their object of interest.
Saying this to someone that "I love you for her" is utterly selfish and the hallmark of a loser guy who cannot muster courage to propose his lady love or in other case, accept the fact that she does not want them in her life. One should never try to reach out to someone who is not eager to reciprocate in the same manner. Expressing your love for someone should not be a desperation move.