Should I tell someone when they are dating a pathological liar?

okay, I know a guy who is a pathological liar and through research and background check I have found out most of what he has said to be lies, like jobs or relationships, his health etc.. He has been arrested for going at an ex girlfriend with a knife for not answering him and at people with weapons and getting into fights, he says he was in the army nope just the cadets and paramedic nope, fire man he just did a course, he was cheated on where he did the cheating, stole from ex girlfriends families, he cheated on his ex girlfriends when he was supposed to be engaged to them and was going around saying he was single guy, slept with two of his half sisters, when these girls break up with him he does a suicide act technique I am not sure if it is for sympathy but when the girls do not come back to him within a day he is fine and within a week on to the next girl he feels no remorse or guilt for his actions. but his new girlfriend wants to be friends with me and I want to tell her but they are always together so I was going to tell her dad all I know as she is close to her father and may listen to him more than me if he is a man with sense and does not want his daughter getting hurt he will investigate what I have said and maybe some things I say will even click with him from what his daughters boyfriend has told her and the family... I really want too tell her father and leave it at well I warned him about this pathological liar and cheater and it is up to her father what he does with the information and how he wants to deal with the situation, but it would be right to tell them as I feel this guy is a sociopath and I do not want his daughter getting hurt as so many girls this guy has dated have come to me with broken hearts from this guy for lying and cheating, I feel like a third party like an accomplice with knowing all I do and letting her and her family feed on this guys lies and deceit. Thank you for taking your time to read and answer I really appreciate it!
Asked Dec 02, 2015

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