What type of mental disorder is this?

My mother wants to know for her reference, in how she should deal with this person - This woman constantly writes on her wrists/arms when she is nervous, hears voices in her head, and is constantly nervous. Based on a sociology course I took before, I am convinced it's schizophrenia... Please share your insight, if any!

Voices - depends on whether this woman hears them from inside of her or as if they are outside. If the voices come from outside, then she may be schizophrenic. If the voices come from within her, then she may be dissociative.
Writing on her wrists/arms may or may not have to do with the voices. She sounds overwhelmed and may write on her body to try to keep track of herself.
Best bet for your mother is to be kind, and to encourage this person to get some help.

Yes, auditory delusions involving voices are usually from schizophrenia.

Schizoaffective disorder.

You are right. These are symptoms of schizophrenia and needs medical treatment fast.

Schizophrenia or a related psychotic disorder. Only during psychosis will a person hear voices.

It is not uncommon for adults to develope Autism. What medications are she taking ? Medication can help but it depends on what she is taking.

She obviously has an anxiety disorder on top of something more serious it seems to be... there may even be a chemical imbalance... has she seen a psychiatrist? It seems like she really should... There may even be some personal or physical/mental problems that she is trying to deal with...

It could be paranoid schizophrenia, yes. If your mother isn't a psychiatrist she shouldn't deal with her. She needs to get her in the hands of the experts because people can throw possibles out at you all day but without a clinical evaluation who would know. It could be a brain tumor disrupting her normal brain function. She could be putting on an act...because yes, people do weird things in the name of attention which would be a whole other problem. Could be OCD with anxiety attacks.

Suspicions affirmative

There are many other factors to consider. It does seem like it may be schizophrenia, but there are different subcategories of schizophrenia. The nervousness leads me to believe that it may be paranoid schizophrenia, which is categorized by paranoia and delusions, although they usually have a fairly normal thought pattern. If this women is somewhat incoherent, and her thoughts don't seem to flow in the normal thought pattern, it may be the undifferentiated or disorganized type. If the symptoms are low-intensity, and she is still able to function relatively normally, it may be residual schizophrenia, but based on what you have said, I'm leaning towards paranoid schizophrenia.

Just so you know, I'm not a certified medical professional of any kind, I'm a pre-med student who works at a Veteran's hospital in the psychiatry ward, and my answer is only based on what I've read and the the experience I've had with patients.

sounds like it to me

She cannot be diagnosed on Yahoo! Answers. She definitely needs to be seen by a Psychiatrist and/or a Psychologist. With her condition being "constant" as you say it would seem that she would need to be seen as soon as possible as she could be a danger to herself. One would at first ASSUME schizophrenia as many have but something you said has to be seriously considered. Since you took the Sociology class then you should be aware of DID (Dissociative Identity Disorder) or what used to be known as MPD (Multiple Personality Disorder). The reason I even bring this up is because you stated that she hears "voices in her head". If voices are being heard from inside the head then that would be different than actually hallucinating. When one suffers from Schizophrenia they hear voices and hallucinate but the voices are heard from outside their head. She does need to be seen by a Professional as soon as possible as they would be able to observe her behavior and listen to her directly and hopefully give her a proper diagnose(s) and start giving her the treatment that is needed so she will be on her way to getting stable and feeling better.