If someone is stopped for DUI and fails a field sobriety test, their car should be impounded.... no matter WHO owns it.
I'd have a problem with that. There are lots of scenarios in which someone might loan somebody their car without having reason to suspect that that person would then take it out on a drunken joyride.
I'd rather have the loaned car considered stolen if you do anything illegal in it. That not only gives you another charge to pile on the drunk, but also doesn't hold innocent people responsible for the actions of others.
I've seen cases where such laws have been enacted, and they're often used (and abused) to take property from people who didn't do anything wrong. There have been a number of cases where civil forfeiture laws were used to take people's houses away from them because unbeknownst to them their (often adult) kid got caught with drugs while in the house. There are countless other cases where property (money, cars, equipment, houses, you name it) is seized "on suspicion that it was involved in a crime," and then the property owner has to prove the property's innocence even after courts have determined that the owner did nothing wrong.
It boils down to quazi-legal government kleptomania, and I'd not be in favor of strengthening the ability of law enforcement to engage in it.