Vaguely directed at Obi:
Were Fister and this lady acting as ideal Christians, they never would have gotten into this situation to begin with. It could also be argued, though, that were they acting as ideal Christians, they may never have started to go out in the first place.
We can suppose that acting as ideal Christians is not the aim of either party involved, so the "rule set" of Christianity can be descriptive, but fails to be effectively normative.
As a Christian, I could give Fister little advice but to seek God's forgiveness for a life of wantonness and profligacy and that he probably deserved infidelity for associating with someone who did not value the act of human intercourse as a sacred union to be saved for marriage. But I will not do this.
As a member of a pluralistic, religion-neutral society, however, I can still offer him advice unclouded by the emotions he's no doubt experiencing that will help him achieve his priorities/objectives. Instead of normatively pushing a viewpoint radically different than his own on him (which he would most likely reject), I can nevertheless offer him courses of action that I see best to fulfill his priorities removed from both my motives and his.
Your religion should empower you and be an asset, not a haze that clouds your perception and therefore (in my opinion) a weakness.
(EDIT: However, as I read more about what's going on, this is less a critique of what you actually said and more a critique of the caricaturization other forum-members are imposing on you, a "hypothetical Obi" more fundamental or misguided than yourself; above all, it's not my aim to offend in this case)
For Fister:
As I see it, Fister, this girl cheated on you for truly base reasons. If you want her for validation, companionship, physical gratification, and a higher sense of self, you should devalue her and cause her to come back to you and want to "make it up."
If you want her for her, out of love, then that love has been betrayed and, for the protection of your emotional stability, you should sever as many ties as possible, harden your heart, and move forward, not looking back.
Cordially,
Lord Tiberius Grismath
1473 for '1337' posts.