"The deceitful comrade does not fight."
Translation:Comes perfidus non pugnat.
I am not 100% sure, but contubernales has a strong war related character (war comrade but also table comrade (so someone you share a meal with)); whereas "comes" is less war related and generally means something like companion (or even teacher). Hence, I would chose "contubernales" in war related contexts, but obviously Duo is more flexible.
I think conturbernales shared a tent, so yes, relates to military encampments. However, since Duo's own dictionary translates both conturbernalis and comes as "comrade" they ought to accept either here. Duo apparently does recognise a distinction sometimes though, as in "Contubernales et comites psittacos vendunt. The tent-mates and the comrades sell the parrots." (naturally, what else would legionaries be selling?)
A "Contubernium" was the smallest organized unit of soldiers in Roman army ( 8 men ), who shared a tent. The civil meaning according to Roman matrimonial law was the illigitimate partnership between a male and a female slave, which wasn't admitted as marriage. "Comes/Comites" is the official title of a member of imperial administration. You cannot use both of them for "comrade/ friend". This would be: Socius, Collega, Commiliito, Gregalis