Not to mention that he could sell those 615 bitcoins for 16 dollars within 1-3 days of buying them at 13 making him $1840 within a week. He could probably do this multiple times. If he bought again using all $9840 at 13 he would have 757 coins and could cash out at $12112 if it hit 16 again. Thus making more than he could ever hope to mine in the matter of 1-2 weeks.
Day trading can be very risky and I honestly wouldn't recommend it with 8000 dollars on the line.
I'd also like to point out that with hardware he has a way to recoup some if not close to all of his money back. If he buys coins then he doesn't get that security blanket.