The right way to lien would be for work done, not storage, that'd be a Warehouseman's Lien, and unless he's in the car storage business and has a contract that might not fly.
He needs to pay for the lien, and pay to get it taken off at the end of the day. Pretty simple to do if you've done them before, but if you're asking these kinds of questions he'll probably need to ask a lawyer.
Then he's got to pay to advertise it in the local paper so the owner has a chance to pick it up, and sell it fairly. Auction, sealed bid, etc. Once you put the ad in the paper the tire kickers come out and will want to know how to put in an offer, for scrap metal value if nothing else. You might be outbid.
If he gets any more cash than he was owed then he'll have to make efforts to return the extra, or sit on the money and hope the guy never shows up looking for it.
It's a right PITA, but it's better than nothing. Places like Kal Tire lien on the drop of the hat, they've got folks that do nothing but, and they're great to deal with. I found dealing with small time mechanics a huge hassle, especially getting liens released.