Is what the questioner did—attempting to distance his female colleague from a Christian man—correct? And what should he do if he comes to love her and she becomes attached to him? And should he marry her?

It is impermissible for a woman to work in a job that exposes her to khalwa (private seclusion) or suspicious ikhtilat (unrestricted mixing) with men. The fostering of relationships between ajnabiyyah (non-mahram) men and women under the guise of friendship or work camaraderie is not endorsed by Islamic law; rather, it is a gateway to fitna (strife/temptation) and corruption. If such a relationship exists between a Muslim woman and a non-Muslim man, it is even more reprehensible, and the claim of inviting him to Islam is fallacious and void. While enjoining good and forbidding evil is commendable, it must be done through permissible means. You must adhere to the boundaries set by Allah and be cautious not to be lenient in speaking with that woman. There is no objection to marrying her if you desire to do so.