“The fact that someone knows the party does not make the comment False”
What makes is false is not the fact that someone knows the party. What makes it false is that you don’t know if it’s true. You stated that the guy is not dating for marriage. You do not know that that is so; therefore it is false, and therefore, it is Motzi Shem Ra.
And please do not say that the fact that he broke up with her is proof that he was not dating for marriage. Many people break up with other people and not for that reason. There can be many, many reasons why people break up with others.