Contingency Fee Question

I recently had a friend involved in a car accident reach out to me about the common fees a personal injury law firm is charging for their services. I’ve kind of always heard a third, but was interested in hearing what others might charge. Also, do you/your friends reduce the fee if the case is particularly strong/easy etc? Is the fee negotiable?

Would like to be able to give my friend some more information before sending them to call a few different law firms. Thanks and have a great weekend!