>>33367804 (OP)I'll give you the benefit of the doubt that this isn't bait.
If you're serious about stealing someone else's woman, you have to do it with conviction and accept that you are doing something immoral - you're not going to do it without looking bad, or at least not if you have to ask how to do it on 4chan.
Assuming they're both happy together and there aren't any glaring problems in their relationship, you have one of two ways to do it. Method 1 is to drive a wedge between them, whereas Method 2 is to make yourself the appealing Option 2.
With Method 1, you can drive a wedge by either making the guy jealous and insecure enough to the point where she grows sick of him and leaves him. If he's not the jealous type or one to have a chip on his shoulder, then this probably won't work, but most men tend to be jealous to some degree. You can do this by befriending the girl you want to steal, grow increasingly present, and cause the guy to question your intentions without raising her suspicions - you want him to come across as the suspicious, jealous type. Alternatively, if she's the church-going Christian type, you play the bad boy and lure her into temptation. Women like to be taken on an emotional rollercoaster - being made to feel happy, sad, angry, or really any strong emotion. In your case, you outrage her to push her own boundaries and be appealing by appealing to her dark side.
Method 2 is you simply become the better, more appealing option than her current boyfriend. You want to worm your way in so that you befriend her enough so she knows who you are and are familiar. Then, wait for them to hit some kind of rough patch or disagreement and exploit it to drive a wedge between them. While they're going through a rough patch, you be the shoulder to cry on but not in a beta way. You be the security and masculinity that she needs which she isn't getting from her boyfriend in that scenario.
Basically, exploit women's emotions and drive them apart.