How do you deal with annoying friends?

I have to meet up with this friend since we've already committed to do some voluntary work together at this org.For some reason she always makes these profound remarks. Ever since she joined one of those paid "self-improvement" forums she's been so high up in her horses, telling everyone what THEY should do abt their lives.She's now always questioning my productivity and makes silly remarks like "oh, what are u busy for?". Apparently she was bothered by the fact that nobody was answering her calls and makes these imaginary claims that she sent me sms when in fact I never got them. Seriously, Im never seeing her again, not just because of 1 incident, but cos she did me wrong for many years,w/ no apologies. After awhile of not seeing her I thought she'd change, but she never did.Im seeing her for last time for this voluntary work,how do I deal w/ her without making a scene! she constantly says something tactless-she has a foul mouth and loves to feel superior!