In their language, it appears that scientists are reluctant to attach the concept of friendship to cats. Although when two cats stay together throughout the day in a range of activities and places, as is the case in cat colonies, it would seem sensible to refer to these cats as friends.
|Cat Friends - by Tim Pearce, Los Gatos|
As expected, cats need to be familiar with each other to be more likely to remain close and allogroom. Allogrooming is mutual grooming - one cat grooms the other cat and vice versa.
When cats are related (e.g. brother/sister, cousins) there is an even greater likelihood that they will be close and allogroom.
Elisa, a regular contributor to this website has her own cat rescue and therefore multi-cat household and she will no doubt confirm the above.
Note: (1) The Welfare of Cats (Animal Welfare) by Irene Rochlitz.