Can't say anything when it's friend's fund. But I will say if it's your fund then you must do research on the project and according to their roadmap, product launch, use case, technical analysis, market sentiment you should set a price target before investing and stick to that.
And in friend's case you could advice him 1) To take out his principal fund and hold the profit amount as long as he is not satisfied. Like he invested $100 and it grew $150. He can sell $100 worth of token and hold the profit ($50) as long as he want but with stop loss.  2) He can sell some percent of the investment and hold the rest with tight stop loss. Like he invested $100 and it grew $150. He can sell $75 worth of token and hold the rest $75 with stop loss.