I like Zentrode's advice here although I'm one to not like breaking people up.
I forsee a problem though... Some girls just like jerks and asshats! I'm sure you know the type they are the sweetest women ever and deserve to be with a great guy! BUT...they aren't attracted to great guys they are ultimately attracted to jerks and asshats.
These girls will talk and flirt with other men. They will talk about how shitty the relationship is because...it's true. Then they will even go so far as even cheat on some of those guys from time to time, but ultimately they end up back with the asshat.
As David DeAngelo puts it attraction defies all. It can trump bad past experiences, logic, common sense, and good advice from friends. If she is attracted to the guy then there is little you can say or do to get her away. She's been with him 5 years for a reason. She may not even realize it herself, but I've seen plenty of girls go through this. Typically when they do become single they'll date some nicer guys and end up back with a jerk. That or the current jerk does one sweet thing and she's back into him.
I'll end my rant, but just making you aware. Whenever I hear someone say she's flirting with them, and she's in a bad realtionship I cringe because I know what's coming down the pike.
BEST OF LUCK!
