Sometimes some people just spin the same game in every session they do, it doesn't matter and it's pretty good if the game gives you a win quite often, but if you play but it's very difficult to get a win or even never give a win by the game you play then obviously they can follow the advice you conveyed this friend, several games or even thousands of other games can be for them to try, It is possible that their luck is in some other game that they have never touched before, and if you do not understand how to play then obviously you provide very good by trying first on a demo account, it is better than you immediately try it with real money but absolutely do not know how to play and maybe it will only cause many mistakes to occur which will also clearly waste more of their money.

For the initial stage, honestly, I would advise them to try the demo mode as you said, because even though they  don't mind losing, what they are betting is money, so the initial statement above as you said would be more effective to try and learn.