What is the ruling on one who says, "I will do such and such, even if it is disbelief and apostasy," knowing that the mentioned action is not disbelief? And does he become a disbeliever by this statement or by his action?

Whoever says he will do something wrong or commit an act of disbelief has committed a great wrong and must repent. It appears from the question that the questioner is suffering from misgivings, and he should turn away from these misgivings and not pay attention to them.

The questioner must fear Allah and adhere to His limits when dealing with non-mahram women. He should not speak to them unless there is a legitimate need, and it is not permissible to be lax in relationships under the guise of friendship or collegiality, because that is contrary to Islamic law and a potential source of temptation. Some jurists have explicitly prohibited speaking to a non-mahram woman without need.