A good friend can't be neutral in such cases. Even worse are those who, like Gina, purport to be neutral but actually take sides.
In my marriage, my husband had an affair with a much younger woman who, like him, was married with two kids. This woman was clearly a gold-digger, as evidenced by the fact that she introduced herself to him by walking up to my pudgy, balding, 55-year-old then-husband amidst a room full of 30-something dudes and declaring, "You are the most beautiful man I have ever seen in my life." Enough said.
My husband left major evidence of the affair around for the kids to find, then warned them, "Don't tell your mother about this or you'll blow up the marriage." Of course, I eventually found out on my own, and the marriage blew up, devastating both kids and everybody in the extended family.
My sister-in-law Patti (wife of my ex's brother) and I had always been close and enjoyed each other's company. When this happened, Patti was appalled and expressed support and commiseration via email. But before long she stopped communicating and I noticed photos on my mother-in-law's fridge of the four of them -- my ex, his new babe, his brother and Patti -- laughing uproariously while splashing down an amusement-park water ride. My daughter graduated from college six months later and didn't receive so much as a card or call from Patti and her husband. The following June, a full year after my daughter's graduation, Patti sent her a "Happy Graduation" card with a note that explained, "I'm not sure when you graduated but I found this card among my stuff. I guess I meant to send it earlier, but due to your parents' messy divorce, I didn't want to get in the middle. [By sending her niece a graduation card?] In any event, I've moved beyond the divorce now and think it's time for you to do the same."
I'll repeat: enough said.