I think the question of DCA vs All-in-buy is largely immaterial from a total return standpoint (Imagine btc goes 10x over next 10 years, will the difference really matter?)

However telling someone to DCA is the smart thing to do to preserve your friendship. Many people can't handle the volatility and they smash buy, btc drops 15%, they sell, then it runs up 30% and they buy in again....etc.

Even though its irrational, they will always blame you for that.....best to tell them to DCA in over the next few months, then if they decide to smash buy its on them.