Pure investment and it will always depend. For me, as long as your friend will not end up broke even though his investment every month will be gone, he will still be able to live after retirement even without the investment (just in case something bad happen). In short, he will not 100% rely on his investment in Bitcoin after he retires.
If he has income from his retirement salary monthly, it is good for his finance and it is better than many people who don't have retirement salary for their life when they are old. If your colleague has good savings, he can use some of it and even includes part of retirement salary in future to invest in Bitcoin or something else. It's just he must afford to lose those money before investing.
Your colleague must consider also if that 20% of their monthly salary of him is too much or not so? Is he willing to lose it every month?
16 years is a very long time, Bitcoin is still 14 years since it was created.
14 years is short when we look back the history but it is too long if we have to live in the market, day by day, month by month and year by year. A lot of challenges in the market and it's hard to live in the market without stress.
More stressful, we can not know about future of Bitcoin and even we believe that Bitcoin won't die, we must have plan for our finance in case Bitcoin will die.