Assalamu alaikum!A 15 years old girl was frustrated by some problems, and she said "Even duas are not accepted and recitation of supplications (to find something lost etc) are all lies". Did she become a disbeliever thereby?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

These words are not a text on belying the Law Maker (i.e. Allah), nor denying the vastness of the Mercy of Allah and His perfect Ability, so that it can be words that take a person out of the fold of Islam.

Rather, the closest meaning of the expression is like the meaning of the saying of the Prophet : “The supplication of one of you is answered as long as he does not hasten (the answer) and say: I supplicated but my supplication is not answered.” [Al-Bukhaari and Muslim]

As for the second phrase, it can be understood to mean that these supplications are not attributed to the Prophet and not accusing the Sharee'ah or the Prophet of lying.

This being the case, then there is no justification for ruling that the one who said this expression became a Kaafir (disbeliever). Ruling someone to be a Kaafir or an apostate is not a light issue.

It is not correct to apply such a ruling on such expressions or other expressions that may appear to be Kufr, as long as it is probable that what is meant by it is something else.

Al-Qaari said: “The expression of a particular person, if it is probable that there are ninety-nine reasons that it could be understood to mean Kufr, and one reason where it does not mean it, then it is not lawful to rule that he has become an apostate.” [End of quote]

Besides, Az-Zarqaani said: “Our scholars resolved that if there are ninety-nine reasons for a word to mean Kufr, and one reason to mean faith (i.e., not negating the person’s faith), then it should be interpreted to mean the best of the two meanings, which is faith. This is a categorical matter and it does not need evidence.” [End of quote]

This is even confirmed for the age of the girl whom you mentioned, as she may not have reached the age of puberty yet. Also, she was very frustrated, and she might have reached the state of extreme anger to the point that she did not know what she was saying.

Allah knows best.