First, no doctor can say exactly how long someone has to live 6 months out. They can only give you an idea during the last month or so. It will all depend on whether your friend's mom has chemo and how she responds to it. It will also depend on whether the metastases are resectable or can become resectable during chemo. There was a recent study done by MD Anderson in Houston, TX analyzing the records of colon cancer patients. They found that the average Stage 4 patient now lives about 30 months. Previously, that average had been about 8 months. There have been many improvements in chemo drugs over the last 8 years and there is a lot of research going on.
You didn't say what kind of doctor told her she only had six months, but my guess is that it might be her surgeon who tend to be the most negative doctors in terms of prognosis. Tell your friend's mom to find an aggressive, competent oncologist and her prognosis might be much better. Stage 4 colon cancer is a very serious disease, but it is treatable.
Jack