I think this could also translate to "The enemy of an enemy is you". Is "friend" better translated as "người bạn" or "bạn bé", or how can we know that we are talking about a general "friend" (or even another person who is a friend) and NOT "you"?
Note that English requires articles despite the absence of classifiers in the Vietnamese.
"The enemy of the enemy is a friend" - shouldn't this be at least as correct as the sentence above? There is no một indicating 'an enemy'. Why isn't it:
Kẻ thù của một kẻ thù là một bạn? or ...một người bạn?
This is why, in some cases, using more 'literal' English translations is better or should be at least accepted.