I don't know what the real number is but coping with a friend or family member with cancer can either strengthen or sever their bonds. Sometimes gaps in a relationship can no longer be covered over while dealing with the more important life or death decisions and treatment of a cancer diagnosis.
It is not surprising to me. Cancer is almost as stressful for the family and friends as it is for the patient. I know of several cases of divorce or separation from online breast cancer message boards. Many of them were relationships that had been hanging on for years and with the added strain of cancer causes the relationship to fall apart. A divorce can be another unwanted cancer side effect.
It is difficult to think that in the middle of a medical disaster like cancer that sometimes the expected emotional support from a friend or family member just isn't there. Boston Red Sox pitcher John Lackey is divorcing his wife while she is in treatment for cancer. I don't think he deserves any blame in this. Its not his fault that his marriage fell apart at the same time his wife was in cancer treatment.
Relationships fall apart for different reasons all the time. A cancer diagnosis can be the last straw that ends a relationship. Also if a friend or family member cannot cope with the fact that someone they care about has cancer and they flee, the relationship will end. The patient is forced to cope with their medical adventures. Any other person is not forced to cope and sometimes can't and the relationship ends. I have friends who are no longer friends as a result of their lack of support.
I was lucky in that my husband was there for me through treatment. I have friends who were alone - one woman went to chemo by herself because she had no one to go with her. During treatment my husband was allowed to be there for everything but was not allowed to see my weight on a scale (the nurses were very cooperative with this). I think our relationship got stronger as a result.
But many are not as lucky. But neither party deserves any blame for the divorce. John Lackey should not be blamed. I find it harsh when the media or others blame someone for a divorce in the middle of a medical disaster. Perhaps the relationship couldn't handle the added stress. But there is no blame here. It just another side effect of cancer - the gift that keeps on giving.