My friend plays online solitaire, poker,and watches youtube.
How will he be affected and how can he continue his favorite online games and youtube?
Thank you in advance,
Youtube doesn't use Flash Player.
If you are still playing games that need Flash Player, then look for new games.
YouTube is not going away. I don't know which games you're referring to but if they are still Flash based on Dec 31st, say buh bye.
Palemoon+Older version of Flash
"My friend plays online solitaire, poker,"
It's up to the game developer to migrate their content to a different technology. This has always been the case.