Perhaps little or nothing will happen to your friend. For sure, if you do not report this accident, you bare all of the discomfort with no hope of assistance.
Driving with an expired license is a provincial motor vehicle offence and your friend could get a fine. Depending on her driving record, the province may suspend her license for a time to bring the rules home.
But, in a rear end collision, fault almost always is against the driver who hit from behind. Fault against the other driver is almost certain and therefore you want access to the offending driver's insurance. That of course cannot happen unless a claim is made.
Depending on the province, your friend could get a fine of a few hundred dollars and if she otherwise has a bad driving record, she could get a suspension of a few months as well.
But, without a claim, you could lose thousands of dollars in compensation including physiotherapy, massage and perhaps compensation for pain and lost work etc..
I suggest reporting.
Consult your doctor over potential long term effects of this accident.
Report it and then consult a personal injury lawyer after hearing from the insurance company and before settling.
Your injury could be much more serious than anything that can happen to your friend.