Howdy William!
I'm currently in a similar situation, except it's a neighbor of mine who's buying that I'm trying to help out. She just refi'd her house in Campbell, CA, and now has a little bit of money to pick up a used Volt - though with a hard limit of $15k. I recommended the 2013 specifically, since it has an improved battery over the 11/12 MYs, gets the HOV sticker, and due to the large number of recently turned in 3-year leases, has plenty of inventory to keep prices reasonable. Check on cars.com (just an example), and you'll find plenty of 2013 MY Volts with very low miles (~30k) around the $16k-$17k asking price. Many/most of these are from dealers, though, but you should be able to wrangle a grand or two (or more if you're a decent haggler) off the asking price to get you right into your $15k price target. For new Volts, the big dealerships (Keyes, Rydell, Capitol, Fremont/Dublin/Concord, etc.) are likely to get you the best deals, as they're moving MUCH more metal than the little guys and are keen to keep their "best Volt seller" titles. For used Volts, some of the little guys outside of the big EV playgrounds of LA Area/SF Bay Area will give you great deals just to get the danged new-fangled electrical car contraptions off of their lots so they have more space for Suburbans, Tahoes, etc.
Assuming you're paying cash, there's almost nothing out there to top buying a gently used, off-lease Volt in terms of bang for the buck. If you're going to finance, there's a lot to be said for picking up one of the few remaining 2015 new Volts still left in CA. The incentives are still pretty good, you potentially (depending on income situation) get the $7500 Fed tax credit, plus the $1500 CA rebate check (just got mine last week!). Inventory, however, is REALLY bad - right now, there are 12 (TWELVE!) new 2015 Volts listed within a 500 mile radius of Mt. View, CA. If your dad doesn't have the income level to get the $7500 tax credit, then it's far better to get used, as that $7500 credit will already be reflected in a lower price. Keep in mind that after all of the incentives and tax credits/state rebates, the net price on a new 2015 (or 2014) is likely to be around $15k before taxes (no options model) to $20k before taxes (fully loaded model). So the effective discount buying used is not very much at all. In other words, Volts really do hold their value well when the actual net acquisition cost is considered instead of the MSRP. That's a great argument for buying new if possible.
One more thing: the 2017 MY can be ordered this month, and deliveries start in March. That may have an impact on 2016 MY prices, so if buying new is a possibility, there may be some added incentives and dealer discounts in the Feb/Mar time frame. Phew - too much to think about. Glad I got my '15 in October! Best of luck to you and your dad on your Volt search. If you and he can nab a '13 pampered, low-mileage, off-lease Volt for under $15k, then you'll have scored an excellent deal. Plus, he'll get in on the ground floor of the EV revolution that will ultimately lead to cleaner, cooler air and some very unhappy oil pumpers in the Middle East (and Texas, North Dakota, Alberta, Russia, Mexico, Iran, Iraq, Venezuela, etc.).