I think that the community onsite here be able to understand that Cancer or any such illness places an enormous strain on a relationship and one or both parties may not feel up to the task of continuuing the relationship.
I think we should not be a "jury of this couple's peers" and "judge them" for not "plighting their troth" or for being married "in sickness and in health" (btw, this is not the liturgy used by all religions as part of a marriage ceremony). This is not (underline not) a Victorian novel. This is life in the 21st century. It is here, it is now. It is Cancer and yes, it's sad. But this man has a right to make a choice. No one should be forced into a relationship that they don't wish to be in - for any reason! It has even been shown that children who are in a household where parents are only staying together for their sake grow up thinking that miserable relationships are the norm.
Mikeysgirl deserves much support for her role in haviing been Mikeys caregiver and it's awful what happened. I'm not minimizing that. She should probably go for counseling, but she should move on.
Relationships often (at least 50% of all marriages fail [whether folks have cancer or not]). These statistics are not improved upon when someone becomes extremely ill. My Rabbi told me (his wife's a Hodgkin's disease patient) that they learned in a support group they attended that when a woman is ill, frequently the man in her life will split (what Carm has said is very true).
A good friend once told me that when one member of a couple feels that either something is wrong with/or they don't want to be in a relationship, that's it, it doesn't matter what the other person thinks - it's true! Those of us who are divorced can attest to that - whether we instigated our divorces or whether they were thrust upon us. Done is done.
It's better to move forward and look toward a brighter future.