My deal with friends is that I try to average all my interactions with them over time.

So, if I have one negative interaction with them, but the majority of my interactions have been positive, then I still consider them a friend—no matter how whacked they may have become.

When the friend-o-meter needle starts tilting into negative territory (i.e. the sum total of my interactions with them is more negative than positive), I begin thinking about cutting them loose.