Mr Nice wrote:Just to make sure.
"It's I helped her" is used in a formal situation.
"It's me helped her" is used in an informal situation.
However, both are gramatically correct.
Am I right?
Thanks in advance.
Definitely not. "I helped her" would be used in formal or informal conversation.
"Me helped her" should never be used. It is using an objective pronoun as the subject of a sentence. "I" is a first person nominative pronoun.
In the initial question, the discussion was about using the objective form as a subjective complement (the noun following a form of the verb "to be".
In your example "her" is a direct object of an active verb and being an objective pronoun, it is properly used.