Think if you let someone be friends, if they cross the line, you need to walk away. It happens that we float along in the passage of time and emotions, but at some point the boundaries maybe discarded, we may feel more relax, then, at that point, the abuser will show their true colors. I think you need to be aware of the transition, and beware of slight behavior changes. If you don't walk away, you have been sucked successfully in. The abuser thinks you won't kick him or her to the curb, but if you keep your eyes wide open at all times, you have a better chance of catching changes if it's truly an abusive person.