Many online writers who've encountered malignant narcissists have described the character traits of toxic people. These include excessive negativity, chronic complaining and bad mouthing others behind their backs.
I'm coming to learn that the best gauge of whether or not someone is a bad companion is simply how you feel after spending time with them. If you're uneasy, uncomfortable, angry, resentful, on edge or emotionally drained, of if you notice yourself questioning your self-worth, then it's highly likely the relationship is toxic.
Sometimes people change, and this change is not for the good. A person who once brought a lot of joy into your life may no longer do so. Sometimes this happens because a highly divisive third party has entered the picture. You can't control this, but you can control whether or not you choose to participate in this dynamic, and all the drama it brings.
If, after thoroughly thinking this over, and realizing there's no other alternative, you might need to move on. I don't believe in suddenly cutting people out of your life, without good cause. However, sometimes you need to do this for self preservation. If you are dealing with a disordered female, directly or indirectly, staying highly involved will only bring you misery.
People and situations don't tend to fit into neat little packages. Someone may not be suffering from a full-blown personality disorder, but keeping in close contact with them might still be toxic for you.