He told his Hindu girlfriend that he would give up his religion; what should he do?
What you have to do is repent to Allah, may He be exalted, say the Shahaadatayn and come back to Islam, because a person becomes a kaafir the moment he says that he will apostatise in the future, or he decides to apostatise, or he says, “If such and such happens, I will apostatise,” because adhering firmly to Islam is essential, and any hesitation concerning that constitutes kufr.
An-Nawawi (may Allah have mercy on him) said: Deciding to become a kaafir in the future constitutes kufr in the present. Hesitancy about whether to commit kufr or not constitutes kufr in the present, as does making it conditional upon some future event, such as saying, “If my wealth is destroyed or if my child dies, I will become a Jew or a Christian.” And approval of kufr is kufr. If a kaafir who wants to become Muslim asks him to prompt him to say the word of Tawheed and he does not do that, or he indicates that he should not become Muslim, or he suggests to a Muslim that he should apostatise, then he is a kaafir.
End quote from Rawdat at-Taalibeen, 10/65
In al-Mawsoo‘ah al-Fiqhiyyah (42/307) it says: The fuqaha’ are of the view that if a Muslim thinks of becoming a kaafir, or he has doubts about the Oneness of Allah or Prophethood or the Day of Resurrection, or he intended to stop being a Muslim, or he is hesitant as to whether he should become a kaafir or not, or he decides that he will become a kaafir tomorrow or in the future, he goes out of Islam and becomes an apostate immediately, because developing doubts contradicts the firm intention to be a Muslim. End quote.
It is not permissible for a man to have a relationship with a non-mahram woman, and it is not permissible for him to marry a Hindu woman unless she becomes Muslim.
You have seen for yourself the consequences of this haraam relationship, which led you to speak words of kufr. Beware and be very cautious of such relationships. Pray a great deal for forgiveness (istighfaar) and do a lot of righteous deeds in the hope that Allah may forgive you and accept your repentance.
And Allah knows best.