At times I have heard my wife encourage people to never, never, never give up on their marriage. This is from a woman who was separated from me for 16 months, and who was unwilling for a long time to work on reconciling with me.
Thankfully God had other plans, and He helped me to hang on when there didn't appear to be hope. "All things are possible with God."
The website that is probably the most helpful for those who want to draw a line in the sand for their marriage is http://www.rejoiceministries.org/. I would encourage such a person to read Bob and Charlyne's testimony, other testimonies and various articles. Dig deeply into what these people have to say.