An egoist will look and treat an altruist wrong. Egoist and altruist , if not diametrically opposite personalities, are completely in different spectrum of behaviors and therefore their value systems and truth to them are completely different.
It depends on whether one believes there is such a thing as a truly altruistic action and what definition of ego one uses. (Medically speaking, this is probably one of the few things Freud got right!)
Probably a "psychological egoist" believes that altruism does not exist. This of course is an ideological stance which depends on a particular definition of terms like "altruism" and "benefit". It is always possible to hypothesize some "benefit" for the altruistic actor -- even one who has sacrificed his life to save someone else! But this does not fit either the phenomenology of the altruist or the functional (evolutionary) explanations of altruism. Altruistic actors may or may not benefit from incurring costs, but the recipients certainly do. Groups that encourage self sacrifice by members (think families, tribes, nations, religious groups) certainly benefit at both the individual (recipient) and collective levels (living in a group where everyone helps everyone else obviously benefits everyone on the average). Humans are pretty clever so they may figure out ways to appear altruistic without really incurring costs, or to get the social rewards of appearing altruistic. But most altruism (defined as doing something that benefits another at a cost to the self) appears to the actor to be motivated by concern for the other. Undoubtedly we may have evolved to have such motivations because of the long-run benefits to shared genes or to the group; but that is not the same thing as finding personal "benefits" underlying the altruistic act.