I'm very sorry for your devastation and crisis.
I think these kinds of affairs are the most difficult to forgive, as they do take some calculation and choice-making by the adulterer. Someone drunk out of her mind who falls into bed on a business trip....never to see the man again.....that type of infidelity seems easier to me to be able to repair.
But what I know of some opiate addicts who are your wife's age: they often have jobs, often with great responsibilities. They often have children, and they show up at the soccer games and PTA. They often go to the gym and work out. On the surface, they look normal. As long as they have their DOC on schedule, they operate in life pretty well, and sometimes for many years like this.
So, if a woman is making apparently conscious choices and behaving within normal bounds in most areas of her life while addicted to opiates, then saying no to sex with another man seems to me something she would be capable of doing.
It will be very hard to trust her again. She has told a thousand lies. I'm so sorry for your pain. You did not deserve this; don't let anyone tell you "it takes two" to destroy a marriage. With addiction, it takes only one.