I would like to note some behavior I've noticed as well as a warning to you Trass..... you have 4 people: (Couple A, and Couple B)
The boyfriend in Couple A catches the A girlfriend flirting with B, he forbids her to do so, yells at her, etc etc etc.
Girl A will then go to Guy B, and tell him about her mean possessive boyfriend after which Guy B totally agrees with her and makes some choice comments. This conversation just made Girl A and Guy B closer in an important way than Girl A to Guy A.
Now Girl A and Guy B have this private thing going between them where it's kind of a game to flirt, and they are in a way "on the same team" after Girl A has been chastised by you. They are more endeared to each other and not only that, but flirting is now more exciting because it's forbidden, and we all know humans, especially women, want things that are forbidden to them the worst....