There are people that will keep having such regret and it will not seize to happen. This is not only about Bitcoin or whatever name of cryptocurrency (people would have held BNB at a very cheap price too like less than $5 but didn't have it again). There are rise and fall especially in the crypto market, futures or spots and that's what makes up the volatility that is what it is.

Your friend don't have to keep feeling like had I know because price will keep going up and down. So he would still feel that way when he sees Bitcoin at $200,000 ? Common... Just tell him to forget about that kind of set back to himself. Afterall, he used the proceed of his sells or profit on the Bitcoin to solve his problem or achieved something that probably he couldn't achieve if he didn't sell the Bitcoin.

He needs to look ahead and understand how the market is a highly fluctuating market and come next halving season, another ATH could be recorded, so he could DCA now to chase the future Bitcoin price and not to keep looking behind.

I guess you have not told him the story of Laszio Hanyecz, the Man that sold his 10,000 btc for just 2 pizzas.

If he is feeling such regret, what does he want Laszio Hanyecz to do?

People should always respect their decision