Shteiger – It sounds to me as if you are violating the issur of lashon hara because you seem to have been mekabel all of these rumors as truth. Even if one hears something l’toeles from a “reliable source”, the Torah still forbids us from believing it. The only thing thing that we may do is take precaution just in case it may be true. The gemara makes this distinction clearly when discussing the story of Gedalyahu ben Achikum. It is important to make a distinction in your mind between what you may know as fact (i.e. that he ended the shidduch with your friend and that he has ended several other shidduchim as well) and what you have heard from others or have come to assume on your own (i.e. he has commitment issues and uses girls to boost his self esteem).
To take any action that would harm this individual by making it more difficult for him to get a shidduch could possibly be a terrible aveira. You are dealing with someone’s life. Please tread carefully.