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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First of all we thank you, brother, for your keenness to prevent the prohibitions of religion, and your enjoining good and forbidding evil. May Allaah reward you for this great act. Yes, you can continue to do what you have been doing, may Allaah increase your zeal. Continue to enjoin good and forbid evil but you have to abide by the Islamic requirements; meaning if your forbidding evil does not lead to a greater evil than the existing one. You have to forbid evil with wisdom and appropriate soft words, by admonishing and encouraging them about the benefit of performing acts of good deeds and reminding them about the danger and seriousness of committing sins. If you are quite sure that you will see the man again whom you saw with the woman, or that you know his house, then there is no harm to postpone advising him to another time when he will by alone, as this is more likely for him to accept the advice. You can give them influential tapes, leaflets and brochures which illustrate in general the danger of being in the presence of a marriageable woman without her Mahram (a non-marriageable relative). Do not forget to supplicate to Allaah to guide them, and do not be despaired from calling to Islam, supplicating, enjoining good and forbidding evil.
Allaah knows best.