People hallucinate when sleep deprived and/or stressed. There could be any number of good explanations. No need to assume or worry about mental illness without other more distressing symptoms or issues. If she is able to work and study without problems, and if you can maintain a healthy five-year relationship, then this is a minor issue.
This may require a look at the stress load and current (if any) diagnosis of post traumatic stress or such. Not to mention, a look at any medications.
Minocycline (sp?) is an antibiotic that can cause dissociation and hallucinogenic side effects, especially in women, and especially if it's gone through half life/sat for a while. I read it is no longer supposed to be prescribed for female acne, as it was routinely, but I have seen it currently in use, so I know there are still Doctors who are not aware of this problem.
If she is on any med, even for some simple condition, she should ask her Doctor if hallucinations is a rare side effect. They shouldn't judge you "crazy" for asking. Even birth control, can cause hallucinations and black outs. It is not listed on the patient education insert, but it is a known and documented side effect. I do not think people are aware that the really concerning side effects are not listed because they are rare, and the pharm companies do NOT want you to know about them.
There is usually a simple answer for everything, but we don't have all the info yet on every food we eat or what is in what we put in our bodies. Think of all the products we use! :)