You don't know any of this to be fact. From what you've posted your W has been drifting from the om for some time and has been looking for an easy out. Besides, two years is an awfully long time to wait for someone when that R is not built on a solid foundation. A lot can happen in two weeks or two months let alone two years.
Also, do not forget that the om is not the reason for your problems. If he was suddenly gone that would not instantly make your M problems go away.
Besides, how much can she really be interested in him if he's about to be gone for two years and she is still spending time with you?
