Must I complete the marriage of a committed young man to a committed young woman after I offered to help him find a wife and then refused to marry him, and is this marriage Islamically valid?

"Acquaintance and conversation between young men and women via the internet or otherwise is a gateway to fitna (strife/temptation) and a means to corruption and evil, even if the conversation is about general matters or matters of religion, as it could be a snare from Satan. Therefore, it is obligatory to cut off communication and conversation with a man, except in the case of assisting in a lawful marriage, while observing the limits of Islamic law and avoiding situations that arouse suspicion."