To determine the grammatically correct sentence, it is important to understand the structure and use of modal verbs and phrases like "had better," which is used to indicate advice or a recommendation to avoid a consequence. The phrase "had better" should be followed by the base form of the verb without "to." In this context, the sentence that correctly uses this structure is:
"You had better tell her everything, or else you will lose a friend."
Let's analyze why this option is correct:
The other options are incorrect due to improper use of verb tenses or structure: