No. It is not illegal to drive without rear window glass, according to the Washington State Patrol, which we asked about this situation.
The Washington State Patrol equipment division told us that there is nothing in the state law that specifically addresses the rear window; however, the driver must have clear visibility of the roadway at all times while the vehicle is in operation on the public roadway.
They said the relevant law is under the Revised Code of Washington (RCS) 46.37.410, where it says (in part):
"No person shall drive any motor vehicle with any sign, poster, or other nontransparent material upon the front windshield, side wings, or side or rear windows of such vehicle which obstructs the driver's clear view of the highway or any intersecting highway."
So, if your friend drives without a rear window or with clear plastic on it, it appears she would not have issues with law enforcement. We'd recommend clear plastic. Rain falling through an uncovered window can cause interior flooding, mildew, mold and electrical issues -- all costly to your car if not taken care of.
If it rains, hails, etc. and the rear window is not covered, then a claim for subsequent damage may be denied due to your friend not using reasonable care in securing the vehicle after the window was broken.
Glass breakage is covered under comprehensive coverage. Since your friend cannot afford her comprehensive deductible, she may want to check to see how much it costs to fix the glass on her own. It may cost less than her deductible amount; meaning that no claim would need to be made for the window.
If she pays on her own for the window repair or replacement, then she may be able to work out a payment plan with the repair shop and can thus get the window fixed now instead of waiting.
If your friend's agent is not able to explain to her what to do in this situation or fulfill her car insurance needs, she should contact a CarInsurance.com agent. Our agents can explain not only insurance terms and claim procedures but also tell her how they can get her low cost car insurance.