... what would you do now?
This is why it is better to never lend anything to friends, give it to them without any expectation you are going to get that money back
If you have real good friends,then there is nothing wrong in lending if you are having money,i was struggling for a while after the recession and one of my friend helped me in overcoming some of my problems and when he needed my help i am there for his aid.
Coming to the OP ,you could figure out a valuation in dollars when you lent the coins and add an additional ten to twenty percent in dollars if he is to return in coins rather than going by the current rate,since you said its your friend you could come up with a good deal yourself.
This is not about having good friends this is about mixing family and business that never ends well and most of the time you are going to lose a friendship over money, there are very friends that have the integrity to pay you back a loan, most of the time they are going to do it very late and they do not want to pay interest, you want to help your friends but you do not want to be a sucker either.