Fire can be either friendly or a foe, depending on the situation. is this a correct paralell sentence

No.

friendly = adjective
foe = noun

Make them both adjectives or both nouns.

Fire can be either friend or foe, depending on the situation.

The racehorse Boundless was not only fast but also a beauty.

Yes, the sentence "Fire can be either friendly or a foe, depending on the situation" is a correct parallel sentence. Parallelism refers to the balanced structure of a sentence, where similar ideas are presented in a consistent manner. In this sentence, the structure is parallel because both options, "friendly" and "a foe," are presented as equal choices in relation to fire.