If a friend ows you 0.1 Btc from Dec 2016 ...

[therhslv]

If you dont wanna lose friends , then never lend them money lol , thats life  lesson trust me . I had few ''friends'' to borrow from me . Now they are gone

[sandwichman1]

if he is a real friend he will accept the refund of the same value of this sum in December 2016

[DrGuns4Hands]

Chase him/her don't let him or her to not pay you because a true friend pays what he/she owes and repay their debt. If you guys have an agreement that he/she will still pay you even the price of bitcoin go down or up that is good but if you don't have one sad to say but it is your fault.

[Jet Cash]

You shouldn't lend to friends. You can give themthe money, give them advice, or take a share in the business. But that doesn't answer the question. If you lent him the payment in Bitcoin, then he should pay you back in Bitcoin. However, you need to be practical. If he has got Bitcoin, and he made it as a result of your loan, then try to talk him into paying you the coin. If it isn't a Bitcoin business, then ask for the money plus interest - say 5%. Value the coin at the time of the loan.

[akram143]

... what would you do now?  

You gave him in BTC so it is better to ask him to return the same BTC in bitcoins the price of that bitcoin raised almost 1600%.No need to ask for interest rate if he is a friend for you.
Next time never give it to a friend if you did just forget about getting back.

[Dailyroberts]

when lending something to someone (in this case bitcoin to a friend) you need some kind of agreement or contract, verbal or non verbal and in that you should set the terms. for example you talk about price and what the plan should be in case it went up or it went down (the risk works both ways). and then stick to it. if you had something like that, then stick to it and if not then that is your fault!

You can't make agreements or written contracts with your close friends. A close friend would have pay its debt anyway. Here is the best thing is demanding the bitcoin amount as 0.1BTC not the dollar value of the time you gave him the bitcoin.

[w5pn73]

But you didn't care about it enough until btc has reached the highs that it has now.
On the other side I understand that it's his responsibility to return what he owes you.

[futuret]

... what would you do now? 

I don't think your friend will pay his loan, it's already a year now and bitcoin price already increased significantly since 2016. One lesson you should learn is, never ever give a loan to him anymore!

You are right it is a very serious issue as bitcoin price is now very high and a lot of people know bitcoin now it is the main reason why he took money and bought bitcoin, it is not good according to me that you should stop helping your friends and not taking interest in any kind of problem he has just give him loan but ask his to be responsible and should pay your loan from all the profit he earned from your money.

[cl37007]

Never lend what you cannot afford to lose is my motto, same with investments really.

It would depend what terms you agreed upon. If you didn't agree terms then he is entitled to give you the dollar value on 2016 as much as you are for expecting 0.1 BTC in today's value

[hndrk10]

... what would you do now? 

Is at sime as it follows: if you gave him 0.1 BTC he owes you 0.1 BTC. If you gave him 20 USD he owes you 20 USD.

Is that simple, he should've known BTC is volatile when he accepted.

[lyks15]

I will tell him to pay me in the same price or value when he borrow me 0.1btc. It is not good to be greedy. Remember that he is your friend not a lending client. But also remember that you are friend not a charity worker so you need to tell him that borrowing is different from giving. And always do a good deed to others specially to your friend because God said,what you do to other people is like you doing to Him. Dont expect any return or interest and let the Lord gave a reward to you.

[petrov trevor]

if it's a real friend it will only ask you 0.1 BTC of 2016, and not 0.1 BTC of today

[kamudd]

... what would you do now? 

So far my friend does not owe me btc, but if it happens to me then I will let it. because I would be uncomfortable collecting it now.

[dhka]

Your case is pretty interesting, I also curious what will you do next
0.1 btc at those time worth not more than $50 I guess but now is around $1500
You should discuss it with him if he is really your friend, but still asking for a 0.1 btc today is too much

[Kezio]

Never lend any form of money, not even to friends, unless they are able to pay it back the next day. And this also applies to you when you borrow money. This is a lesson I learned the hard way, never money.

[Svelto]

Since you are asking about how you should price the 0.10 BTC. My opinion is to price it at its current level. Because if you don’t lend the 0.10 BTC and hold it till now, your 0.10 BTC will worth the current market price.

Since your friend borrow in BTC, he should return in BTC. Fair enough.

[BartS]

... 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.

[EllaPonchik]

Would ask him to return it of course. This is a very good topic and depends on whether you gave him money in bitcoin. And if not, then you should not talk about it.

[MakersONE]

I advise your friend to hurry. His debt for the past year has grown immeasurably. If he procrastinates, he will have to sell the apartment. 

[Fancyax]

It depends on your friend, if he is honest he will pay back your  0.1 btc even after years no matter its value.
Good luck with your friend.