Avastin stops or slows tumor growth in metastatic colorectal cancer by preventing the growth of new blood vessels. It is an adjuvant therapy. Xeloda slows the growth of tumors. Both drugs have only been in use for a relatively short time (since about 2005), so there isn't any really long term use experience with it yet.
Stage 4 may only mean that her cancer was metastatic, not that it still is. If her scan doesn't show any tumors or cancer cells sufficient to show up on the scans, she may in fact be cancer free at this time I am assuming the scan you are referring to was a whole body scan, and not just of the area that was surgically removed. If that is the case, these drugs should reduce the risk of recurrence. However, her oncologist's comment that you posted makes me thing that is not the case, so you may want to get some clarification on that from her oncologist.