I agree with Clydesmom, that your friend's right to reject the settlement would by subject to terms of the contingency representattion agreement. Nevertheless, the plaintiff and not the lawyer is the only party that should be able to sign the dismissal order or the tort release.
In the event your friend can get the settlement agreement rescinded, your friend would probably owe the original attorney an amount equal to the contingency agreement plus the legal expenses incurred.
You have described an accident with light vehicle impact dynamics, with injury diagnosis made by a chiropractor. You similarly describe a settlement yielding your friend $5000 after lawyers fees.
The value of your friends injury claim would weight heavily on the actual clinically objective diagnosis of the injury. Additionally, since you describe a light impact accident, the burden of proof would rest with your friend to demonstrate that the accident dynamics caused the injury.
If there were objective suggetions of permanent injury, it would appear unusual that the chiropractor didn't refer your friend for evaluation by a medical doctor.
There is nothing wrong with researching the value range of the specific injury. Factors that would affect such value would include, but not be limited to trial venue, demonstrable clinical findings, reputation and expert testimony results of the chiropractor, and witness credibility potential of your friend. Many of the factors are archived by venue specific jury verdict services. It is possible that your friend's attorney subscribes to such a service and has already conducted this research.
Regardless, your friend's lawyer should take the time to explain how the settlement amount was negotiated.