How do you deal with an employee who doesn't get along with the rest of the staff?
Often, workplace dynamics lend themselves to demonizing or scapegoating one employee. Having one person on the outs helps the rest of us to feel connected. An evolutionary psychologist might explain this as a leftover from a time when humans counted on the tribe for survival. A developmental psychologist might point to adolescent norms - think about high school in-crowd stereotypes.
Typically, workplace conflicts begin with someone feeling dismissed, discounted, disenfranchised or disrespected. The trigger for this feeling may be an actual slight or just a perception. Next, the person who is triggered, responds. Sometimes that response will be a confrontation and sometimes it will be of a passive-aggressive nature, like avoiding the others at lunchtime. Then things go back and forth, back and forth. Each side steps on the others toes. Eventually, both sides have long lists of atrocities that have been perpetrated.
The cold-war that you describe is hurting productivity. Who is in charge in this office? Would management consider bringing in a mediator? A wise leader will investigate and either mediate a truce or find some other solution. It may be the American way to either compete/combat or avoid conflict. However, in the workplace, where on-going relationships determine our success, conflict management strategies such as compromise, collaboration and accommodation are much more effective.