Lets flip the script. Let's say your friend owed you .1 BTC from Dec 2016. Now fast forward to 2018. BTC has crashed and the price is now 10 cents apiece.
Would you be ok with your friend saying "Hey Bro, here's the .1 BTC I owe you..."
I'd say he should pay you what ever the price was at that time.
His friend was a trader. He should have been be aware of the potential opportunity cost that he was taking away from the OP by borrowing the 0.1 BTC for a year. Unless there was a specific agreement stating otherwise the friend should repay the asset that was borrowed, not the cash equivalent from a year ago.