Ifan does not notice if you kill Alexander outside of his dialogue. So if you say "Wait a sec, we might need this guy" in the Ifan-dialogue and then a bit later decide you do not need this prick anymore and smoke him from the face of this earth, Ifan does not comment or notice.
He even later in the game complains in his story dialogue we did not let him fulfill his vengeance (even though he absorbed Alexander's soul after killing him).
Ifan should notice if you kill Alexander as long as he is where you also would kill him if you you let Ifan do it in the dialogue.
