Is my friend held accountable for cursing God in her mind, knowing that she does it involuntarily, seeks forgiveness, and feels despair because she cannot stop, and is this from the work of Satan or from herself?

Obsessive thoughts and self-talk are forgiven as long as they are not acted upon or spoken, based on the Prophet's (peace be upon him) saying: "Indeed, Allah has forgiven my nation for whatever they obsess about or talk to themselves about, as long as they do not act upon it or speak of it." A servant's dislike of these thoughts is a sign of sound faith, as the Prophet (peace be upon him) said: "That is pure faith." What your friend experiences is from Satan, and the Prophet (peace be upon him) praised Allah who limited Satan's cunning to mere whispers. Striving against these whispers earns its doer a reward.