These answers fascinate me. You really perceive no difference one way or another? "He beat somebody up" so he's equally bad and his motivation and who he chooses to beat up makes no difference? Really? In scenarios 1 and 2, beating up your girlfriend is no more or less ok than beating up a total stranger? Really? Or, as both Reelya and IronyOwl point out, the guy she slept with had presumably made no commitment to anyone, whereas the girlfriend presumably had. Do you really perceive no difference between his choice of target? Retribution against someone who breaks a commitment is no more or less ok than directing your retributino against somebody totally else? Seriously? If Bob promises to do something, and then doesn't deliver on his promise...it makes no difference to you whether I beat up Bob for his failure or beat up you for his failure? Seriously?
Consider that while in scenarios 3 and 4 we can probably assume that the friend knew about the relationship, in scenarios 1 and 2 there is no indication that the guy she slept with had any knowledge of the relationship at all. Imagine being the guy in that possibility. You have consensual sex with a girl, and then some guy you don't even know beats you up for it. Now compare that to being the girl. You cheat on your boyfriend and he beats you up for it. Do you really perceive no difference between the two?
You're privileging the worth of one of the individuals he targets over the other for arbitrary reasons which weren't even part of the situations you framed. I would agree that if there was an explicit agreement of exclusivity between the boyfriend and girlfriend, he would be more
justified in seeking retribution against her than against a stranger with whom he had no relationship, and likewise than against a friend (with whom he presumably didn't have an explicit agreement of similar sorts).
However, that was not part of any of the scenarios you presented. Literally all we know about the situation is that the boy and girl were in a relationship, and the boy perceived it in such a way that he considered the girl sleeping with someone else 'cheating'. There's no explicit agreement of exclusivity -- you might argue that such a thing is considered to be implied, but open relationships exist and are even somewhat common, even in some cultures which traditionally promote exclusive relationships.
Furthermore, if you
do wish to presume that there is an implicit agreement of exclusivity between the boy and girl, created by a culture and society which treats exclusive relationships as the norm, then we must also assume that there is a similar implied agreement between the boy and his friend such that neither will infringe on the exclusivity of any relationships that the other may be involved in.
If that is assumed, then scenarios 2-4 are identical in terms of the justification for the boyfriend's actions, with scenario 1 standing out because of his relative lack of justification for assaulting an individual who may have been unaware that the girlfriend was in a relationship (though obviously if he was aware, scenario 1 is equivalent to the others, as you're apparently assuming an underlying societal agreement to preserve the integrity of exclusive relationships).
The thing is, all of that is related to the relative degree to which his actions are justified, or in other words, whether or not he has good reasons for assaulting another party. None of that pertains to the moral rightness of his actions. In every scenario his assault on another party is equally wrong because
causing harm to other people is wrong. The girlfriend and other man are not in the right (save perhaps the stranger, if we assume that he did not know of the relationship, but that is not given as a condition for that scenario), but they are less wrong, and in any case the magnitude of their wrongful actions do not affect the magnitude of the boyfriend's own wrongful action. The justification or lack thereof is irrelevant; society does not allow him the right to physically damage another individual in any of these cases.
Though there perhaps is an argument that he is less morally wrong in scenarios 3 and 4 because he is willingly removing himself (and thus his penchant for causing harm to others) from the life of one of the other individuals.
I truly hope this isn't going to turn into some libertarian social Darwinist fantasy where he was right to beat up his girlfriend for cheating on him because he has the capability to do so and feels that he has been wronged.