16 years of saving 20% of his salary into BTC may be a wrong and risky move because of the uncertainty. I feel he should not dive into this if the saving is really towards retirement.
Although there is a risk behind this, I don't think it is the wrong investment due to the future we see in Bitcoin. This is a great plan, and I am sure he will benefit from it.
I will also suggest that he can do like 5% in BTC for the duration of 16 years(Take the interest as often as possible) In my own opinion,20% is too high. However, if the retirement plan stated above is not available in his workplace then he should invest 15% out of the 20% in a more secure investment because he's saving towards retirement while 5% goes into BTC.
20% in BTC investment is not a poor idea, nor is it a risky investment, as you suggest.
This demonstrates to me that you did not trust Bitcoin, which is why you are concerned about investing 20% of your salary for the next 16 years.
Remember, the employer should have a fixed retirement savings account; hence, if he risks investing 20% of his salary for the remaining period, he has a very good strategy to have enough money to spend after retirement.