Selling your bitcoin in order to invest in something else to me, is not advisable, considering the general situation, especially the coming of the bitcoin halving. The price of bitcoin will rise again and possibly go above $69000 and more, so there is hope of making good profit at the end of the day. Unless if you need fund for very important and beneficial project, then you can sell a little reasonable and suitable amount, if not, l suggest you still hold on.
the best thing that he need to do is holding it until the next bullish market trend, that's was good advice. I personally will try not sell any of my btc for anything until it's price reach the peak at bull run. I think it will be better for him to make a plan before investing, maybe it will be good if he divide his fund into some part so he will not touch his investment fund for btc so he will not sell his btc in lower price.
Always think long-term, not short-term, because you tend to sell it off early. That is really our goal: to sell it off after a few years because that is where we see a profit. Though the problem right now is that we tend to get emotional when there are dump, like when we think to cut off our losses because we are thinking that it will drop more, but it is just a short dump. Again, it may be easy to say that you will sell off the next peak, but the problem is that if you can overcome the obstacle of holding it, meaning there are dumps along the way, are you not tempted to sell it off, or you think that it was the peak but it turned out not to be?