You might want to discuss this with her. Does she know it happened to her? Was this the first time? What happens to her after an orgasm during masturbation? Was her vision affected?
This is one of the times when I think it is better to err on the side of caution and see a doctor. Just to make sure it is nothing serious or a small stroke.
It can be just a natural reaction to the high levels of hormones released in her brain and blood after orgasm. Or she did not breath on her way to the orgasm. Her body goes through a lot of stress and tension during sex. Especially the slurred speech is a sign that she should see her doctor to make sure things at ok.