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.