I've got this friend…
Some people with eating disorders like to be praised for successful meals. If your loved one didn't binge or purge or ate an appropriate meal, then praise them if they find it helpful. Don't assume you know what he or she prefers.
Give the gift of breathing room, if needed. If your friend or family member needs to be away from the group and go for a walk or tune out with his or her iPod, then trust them to know what they need.
Most of the year, if you are concerned about a loved one's disordered eating, then it is an appropriate response to initiate a kind, loving conversation about your concern. When the family is gathered around the Christmas dinner table is not an ideal time. Wait, then reach out.