@Writerfromanotherstar Sorry but i don't agree with you about homewrecker is wrong for the reason i mentioned previously. As i said before, the homewrecker does not have any obligation to respect your relationship. Being in a relationship only makes you entitled for such respect from your partner. But it does not magically makes you entitled for such respect from other people outside of your relationship. Sure, it would certainly be nice and the moral thing to do for other people to respect your relationship but it would be ridiculous to demand such respect from them when no such obligation is owed. Being a home wrecker probably would be an immoral thing to do in most cultures and would probably makes you a bad person. But being an immoral person alone does not makes you wrong.
You do not need consent from other people to enter into relationship with your boyfriend, no? Likewise, those other people also do not need your consent to enter into relationship with your boyfriend. Your boyfriend still needs your consent though because he have the obligation to respect his relationship with you. And thus when your boyfriend cheats, i think the only one that is at fault is your boyfriend because he does not fulfill his obligation towards you.
If your friend is the one that goes with your boyfriend, sure you can blame her too if you think being friend with her entitled you for her loyalty to not touch your boyfriend. I don't know and i cannot tell you what to expect or what not to expect from your friendship with them.
Think about it like this. When you have a contract to works in a company, you have obligation to fulfill your contract and works for your company. But other company does not have any obligation to respect that contract. Other company is free recruit you to join their company as they wish. You would be wrong for breach of contract but the other company is not.
Tl'dr:
No obligation = no wrong