From my experiece i have never known them to overturn a decision to df. You can try but i doubt that they will do so. Once a decision has been made they usually stick to it.
This and several other comments on this thread are based on misinformation. In fact, appeal committees can and do overturn the decision of the original judicial committee. It doesn't happen often, but if there are mitigating circumstances and the appeal group determines an error in judgment was made the first time around, it will be overturned. I was on such an appeal committee.
Of course, absent knowing all the circumstances of this case, it is impossible to predict the outcome of an appeal. Here's what will happen if she does appeal (within the 7 day limit, although if she calls and says an appeal letter is forthcoming they may bend that a bit). The appeal letter, by the way, does not require any "help" in the writing -- Just write, "I am appealing the decision to disfellowship me," date it and sign it and get it in the hands of one of the guys on the JC.
A brand new committee will be assigned by the CO within a week or so, and they will call a meeting, at which the appealing party will be face to face with at least SIX eldlers, the original AND the appeal committees. She will be told that the only grounds for appeal are that "a serious error in judgment was made" and asked what that error was. If she doesn't have a good answer prepared, it's all over.
In summary, the biggest job is not the writing of the appeal letter, it is the getting of your act together so you can cite "mitigating circumstances" that led to the original committee making an error in judgment. You will have to show you were "repentant" at the time of the ORIGINAL committee meeting (if you got repentant later, they will dismiss your appeal by saying that was the whole purpose of the DFing, to make you feel the repentance you failed to show in the first place).
The fact that so many wrote in with misinformation about the whole process confirms my long-held theory that this is a kangaroo court operation. I always thought an elder ought to be appointed to represent JWs who faced a JC hearing, to apprise them of their rights (a form of "defense counsel") but when I suggested this to a CO l used to know, he scolded me for having the wrong attitude.