My friend is dating a police officer, need help?

A very close girl friend of mine is a very sweet but can be sometimes naive. She is in her early 20s and recently complained to me that she wishes she had a decent guy to fall in love with. I understand there is a shortage of good men and she isn't attracted to bad men by any means but she has been dating around to see what she likes and has started dating a police officer. Now i know not all cops are bad people, sell drugs, take steroids or are all violent against women- but her friends, family and i are very concerned that she may slowly be pulled away from her loved ones and get caught up with a bad crowd. I already saw a few red flags last night at a party when he was being domineering to her and I made the mistake of making a comment to her to not let him talk to her like that and she reacted negatively. Her friends and family are all very worried about her; is there any literature or advice for somebody entering a potentially poisonous relationship? Any feedback would greatly be appreciated, thank you very much.