By telling them to invest only the money they won't need in the next five years, you are implicitly promising that they will get the invested money or more after that period. I wouldn't do that to be honest. No one knows what will happen in or after five years and you don't want to be held responsible if something wrong happens and they lose their money.
By telling them to invest only what they can afford to lose, you are preparing them mentally for the worst case scenarios.
Exactly,and also,before investing,please make sure you do proper research on the particular coin you want to invest in,because if you just invest in any coin,there is every tendency that you might end up losing your money,therefore in order to avoid such scenario,always make sure you read more about the coin that is doing well at the period of time,or the coin that has the potential of doing well in the nearest future,so that you can always benefit or gain.And secondly,invest a little,do not use all you capital to invest in a particular coin.