Is a Muslim who helps another Muslim to apostatize deemed an apostate or a committer of a grave major sin?

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

First of all, thank you for your concern. The question is somewhat vague and ambiguous. However we hope that the following answer will be sufficient.

Scholars maintain that being content with apostasy or disbelief is itself an act of apostasy. They support their opinion with verses from the Quran and Hadeeths from the Sunnah of the Prophet, sallallahu ‘alayhi wa sallam . For example, Allaah The Almighty Says (what means): {That is because they followed what angered Allaah and disliked [what earns] His pleasure, so He rendered worthless their deeds.} [Quran 47: 28]

As for examples from the Sunnah, it was reported that the Prophet, sallallahu ‘alayhi wa sallam, said that a person who is absent from the place where a sin is committed but is content with it incurs the same sin as the person who is present at the place where the sin is committed; and the person who witnesses a sin being committed but denies it and is not happy with it is similar to a person who is absent from the place of the sin and rejects it. [At-Tabaraani in Al-Kabeer]

Furthermore, Shaykh Al-Islam, Ibn Taymiyyah mentioned that a person who is content with his own disbelief and that of others, his own dissoluteness and that of others, and his acts of disobedience and those of others, is not  one of those who pursues the contentment of Allaah, nor does he believe in Him. He will be exposed to the wrath of Allaah and His curse and punishment.

If mere contentment with the disbelief or apostasy of others is considered apostasy, then helping a Muslim to apostatize or to encourage him to do so is an act even more worthy of being described as apostasy.

However, the questioner should know that this explanation is general and must not be applied to a specific person. This is because judging a person as an apostate must be done by virtue of a judicial decision. Muftis, callers to Allaah and common Muslims must not pass such a judgment because it has to be based on investigation and careful deliberation, due to its serious results in this world and the Hereafter. Indeed, describing a Muslim as a disbeliever or an apostate is a serious matter. In this regard it was reported on the authority of Ibn ‘Umar and his father, that the Prophet, sallallaahu ‘alayhi wa sallam, said that a person who calls his fellow Muslim a Kaafir (apostate/disbeliever) must either be actually so or the perpetrator will incur the grave sin of this description. [Al-Bukhari and Muslim]  

Allaah Knows best.