And what exactly does she feels she owes to you - or to her husband, never mind boy b!
i would say she does not need anyone there for her as it's high time she took responsibility for herself.
No it's not easy to walk away and you can't turn feelings off, but you CAN be realistic and you CAN protect yourself.
You know that walking away from her is really the best option.
You acknowledge you have been weak, now start being strong.