Can't help much on the price issue but thought I'd help out on the convincing.
If he buys a Mini he's stuck with a basic rifle. He might be able to add a scope or something but he's limited to what he can do with the rifle.
If he buys an AR-15 he has many future options that can be built off that one lower receiver.
He can buy a short handy carbine today then if he later decides he needs a 20" rifle its a simple matter of buying a new upper.
In fact his options are limitless:
He can buy upper barrel lengths ranging from 16"-24".
He can get carry handle uppers that include iron sights or flat top uppers to mount optics.
He can get many different calibers:
223, 7.62X39, 45ACP,40S&W, 9mm, 9X23, 10mm, and I'm sure I'm forgetting one or two.
I think there are .50BMG uppers available.
If he decides he wants to try Highpower rifle competition he can use his AR-15 and very competitive.
There are 10X's as many vendors selling aftermarket parts for the AR-15 than for the Mini.
As c-rock stated the price for quality magazines are very far apart. Used factory mags for the mini sell for over $100 each. You can easily buy 3-4 AR-15 mags for that price.
If all that doesn't convince him tell him that Bill Ruger shares a large bit of the blame for some of the anti gun laws, why would he want to support gun control?