If I agreed, I'd suggest planing the meeting in your office at a time convenient to you. That way, if she doesn't show up, you can still be productive and not waste the time. I would feel no obligation to go out of my way to meet someone who stood me up when I was doing them a favor.
I would do this and schedule her at the end of my workday so neither her presence nor her absence interfere with my workday (so if you work until 5pm, then schedule her at 5pm). I'd then tell to contact me right before she leaves so I know she's on her way -- e.g., if it takes her 15 minutes to get there, she needs to contact you at 4:45pm. Let her know that if you don't get confirmation, you'll conclude she is canceling so you leave at 5pm like you would anyway.
It might sound harsh, but I have found this is the only way to deal with individuals who either a) are perpetual flakes and/or b) are perpetually tardy. If she asks why the stringent schedule, you could let her know that you have a tight schedule (even if "tight" means, you have very little time to deal with BS) and because she has continued to cancel or no-show, you need to know for sure she's going to make an appearance (and no calls at, say, 4:45pm saying, "I'm leaving in 15 minutes" because that means a) you end up waiting and b) she may change her mind again so you end up waiting for nothing).
If you're happy to help her, I'd mention that also, but I'd be honest with her (if she asks) and say, "I'm happy to assist you with this issue but because you keep canceling or not showing up, this is what I need from you to make this happen." Again, I had a friend pull this kind of disregard for my schedule when he asked ME for a favor so when he needed to reschedule (he needed me to drive somewhere so he could load a big box in my car that wouldn't fit in his), I told him, "Go to the store, buy what you need and, once you're done, THEN call me, and I'll drive out there to meet you" (because I wasn't willing to wait while he window-shopped for other things and because I know how tardy he always always ALWAYS is, so I wasn't going to spend any more time than necessary after he already screwed me over without apology when I set aside time to do this favor for him then he flaked -- his flakiness disrupted my AND two other individual's schedules).
If she really wants your help, she'll do it the way I suggested. If not, she's on her own, AND you don't have to waste any more time on her inconsiderate behavior.