If porn is something she simply cannot tolerate, she should stick with her sense of that. It is an extremely common vice, but it is also a very difficult one to eradicate, even for those who want to give up this awful habit. If her fiancé does not see both the infidelity and disrespect in thought that porn use represents, not to mention the duplicity in looking at it secretly when she had made her feelings about it clear, then I think she would be wise to call off the engagement. I say this because he sounds very likely to continue to betray her by indulging in this vice secretly. If that is something she can’t live with, she shouldn’t marry this fellow, because he has essentially told her that this is part of the package if she takes this particular human as a spouse.
IOW, it is not that she caught him doing it, but that he did not say, as he would if he had violated some other moral law that he considered part of his own moral code, “I am sorry. I should never have done that” rather than “I am human.” Every thing a soldier must be requires mastery of his lowest human inclinations, which surely would tell him to flee from harm, including the natural consequences of his actions, to work when he felt inclined to work and rest when he felt inclined to rest, and to indulge in whatever pleasures he felt inclined to indulge in when the opportunity presented itself. A soldier who abandons the battle field, when confronted by his fellow soldier about it later, does not say, “I am human.” He says, “I failed you in your hour of need, brother, and I make no excuse for myself.” Even then, he knows he deserves less trust than he did before, that he will have to earn his way back into the esteem of his comrades. This fellow does not seem to have that attitude about porn.