What is the grammatical error in this sentence he would not have fought them if they followed his advice?

The grammatical error in the sentence "He would not have fought them if they followed his advice" is the incorrect tense of the verb "followed".

The sentence is written in the past perfect conditional, which means it describes a hypothetical situation in the past. The verb in the "if" clause should also be in the past perfect tense, which is "had followed."

Here's the corrected sentence:

He would not have fought them if they had followed his advice.