It depends. Since you signed a rental _contract_, letting somebody who isn't authorized by that contract drive your car might be considered a breach of contract, which is a civil matter. Of course, depending on the case, there could be other issues, such as theft or such, that might be brought up.
The worst that could happen? Hertz could sue you for a contract violation (unlikely). They could and probably would add you to their do not rent list if they found out.
It's a risk vs. reward thing. The risk is probably rather small.