having just read thru your post & the 3 replies i think in your situation i’d combine all 3.
first talk to her sympathetically even if it has been a long time since your friendship went cold. like rabbitpam mentions, if you take that approach you may give her opportunity to come clean about something thats going on with her. clearly something *is* going on with her. and this would be the least risky approach imho because it can remain between the 2 of you (officially anyways… not that she couldn’t go & talk about it with others, but in that case i think it’d be difficult for her to make you look bad). but she may not be willing to admit it or she may not even be aware of it herself. i agree with the possibility that she’s self-sabotaging… i’ve seen that happen often when some one gets tired of their job. then they can eventually leave with the ‘good riddance’ attitude and feel no regret.
if that falls flat or worsens her disposition towards you then i think it’s worth a trip to HR. especially if her behaviour includes disparaging you. altho i say this never having worked in a company big enough to have an HR, lol, so i may be oblivious of the politics involved. does your company have a performance review, or do you have an immediate manager who regualrily monitors your work? i think an important thing is to be protective of your own job performance and have it recognized. sort of a ‘watch your back’ type thing so that she cannot dis-credit you and put your job at risk.
an alternative or follow up to the other options would be to ignore her as much as possible and just chalk her behaviour upto her childishness.
the last office i worked at before i went freelance had a couple of employees, in departments other than mine, who i had to interact with and who were verrrry difficult personalities. i worked in the design department which is always the bottle-neck department… always working on a highest priority job as determined by my boss with other jobs breathing down your neck. basically, 2 people in particular, would throw tantrums if they couldn’t get what they wanted fast enough. literal tantrums. yelling, foot stomping, arm flailing, paper throwing tantrums. at first i was shocked and freaked out by the behavior, but i soon realized that this was what happened in the big board room meetings with the owner of the firm so they there was basically nothing i could do about it but cope. in my situation i wasn’t the person calling the shots anyways, so i’d deflect the problem to my boss and ultimately, whoever threw the biggest tantrum with my boss got their way. needless to say i didn’t stay there very long. my blood pressure is high enough anyways.
it’s a good thing you don’t have to work closely with this gal. but if she contiues to be a biotch then find the best way to ignore her, and make sure there are people around you who can vouch for the fact that you’re doing your job well. good luck!