Spiders are common in dreams.
Here is some info about
spider dreams.
I also think that the spider seems to represent something that she is avoiding/repressing. She knows that she will have to face it - the spider biting her - but she is afraid of the consequences (her feeling like she is dying in the dream).
Dying can be a positive sign in dreams though, because it represents change and new beginnings, so the dream could also be saying that she has to face this and deal with consequences so she can move on.
Is she OK physically? Could the part about her not being able to breathe have a physical cause?