First off I wouldn't call her a friend.....she is really just a toxic nuisance ACQUAINTANCE. When involved in things whete she is also at, just go about your OWN experience & don't give her the time of day.

If she approaches you after someone left who was talking to you just say excuse me I was heading to talk with someone.

If she is in the hallway after you get out of the bathroom just walk past & go where you were planning on heading in the first place. You can nod to acknowledge her but just keep walking & if she tries to stop & talk to you just say excuse me I need to get back to the people I was talking with.

Avoiding her by constantly excusing yourself from her company when she shows up.....it may actually sink in to her if it keeps happening consistently.