This should be the best idea I have seen so far as regard to the question asked by the OP. 1 BTC will always be 1 BTC as long as you don’t touch it or spend it for something else. The value of 1BTC today will always be the value of it in a few years to come as long as you hold on to they your BTC. Due to the volatile nature of bitcoin, it is better you lend him bitcoin as a currency and he should pay back as bitcoin too as a currency. With this idea, the OP won’t lose anything from the savings he has accumulated so far. I hope he makes the right choice about it.
Thank you for this suggestion to borrow the with Bitcoin to avoid losing the value of the amount borrowed, but then we have to think about it two ways which is if the value of the Bitcoin decreases from the Dollar value at the moment, let's say Bitcoin correct back to 30k at the time he wants to return the Bitcoin, at that point, although the volume of bits paid back will be same but what of the DCA value.
I have already loaned the guy some amount in my local currency but under the agreement that he pays back with interest, and at stipulated time frames, that way if he defaults from the timing, the interest will help to protect my capital loan amount that is in his hand.