I know it is incorrect to say, "They helped to her" and that it should be, "They helped her", but why is the word "to" not needed? And yet the word to is in this sentence: "They helped to get her free." Why is that?
Because help (like want and force) takes a direct object for the person or actor and optionally an infinitive clause with to.
[That is a more detailed version of because it does, some form of which is the whole of the answer to almost all questions about why languages are the way they are].
help takes the direct object without any preposition.
help is used with the preposition to then it is followed with the verb.
Here is the sentence with the direct object and the verb:
He helped her to feel the pleasure.