It depends on the rental car company.
The additional driver fee isn’t meant to be reasonable. I think it is a disgusting way to make extra money. However, unless it is a bona fide emergency, It isn’t a chance I’d take.
Then again, I’ve seen too many messy situations from my 2 years working rental car customer service issues. Many of the rental car companies are hurting financially and they’re hungry for every nickle they can get.