Such situations are bizarre. I am following to learn more though. All forms of sacrifices to save the many of the human family may not necessarily be stupid. Military persons are groomed to prioritize the lives of citizens though. To die for others, its a courageous, selfless and humanly attitude. However, this should be done when that is there is no alternative to save every life!
But from life experience I can affirm that being outside a person you love and if that person turns out to be the most egosite in the world, must be prepared to fall and suffer because of the selfishness of that person.
It's easy for a psychological egoist to regard with indifference or else write off the self-sacrifice by others as stupidity, derangement, etc., or even applaud it if it benefits the psychological egoist himself. Or a psychological egoist not inclined to armchair psychologizing might simply say, "Well, if that what they want to do, they should do it. Different strokes for different folks." And who's to say those folks might not be psychological egoists too?
So perhaps you should ask whether a psychological egoist could, in principle, consistently (1) commit an act of self-sacrifice or even (2) commit simple suicide without self-sacrifice. I think the answer in both cases would be yes. If the psychological egoist's interest in others is entirely self-regarding (perverse, but still possible) he may see no point in continuing to live without those others; likewise a psychological egoist may simply desire and choose to end his existence (perhaps in disregard of those who care for him) because his other desires can't be satisfied (this need not be perverse, if e.g. great pain and suffering are involved).
Depends on how narrowly or broadly you define 'egoism'. Understood broadly, it means that the individual acts only to alleviate own actual or anticipated suffering (even at the low level of mere discomfort) or to enhance own actual or anticipated pleasure. Add to that the strong evolved tendency of humans (as among the most social of animals, including a high level of communication) to be indoctrinated with beliefs (about honor and dishonor, and often about a rewarding and punishing deity), and acts that are objectively self-sacrificing do not seem so, on balance, to those heavily thus indoctrinated. Non-egoists such as I suggest different motives for at least some individuals, but that is a VERY long story.
Actually, there is an easy way for a psychological egoist to explain such acts of self sacrifice. They just might be selfish acts. For instance, if a person believes that such an act is rewarded in the afterlife.
The other way to explain such an act, although in this case it would be more with respect to a sacrifice to save loved ones, is that the selfless person might find it impossible to live with himself, if he did not make that sacrifice. In a sense, it is a selfish act.