You have to define a "serious game" before you can define a "friendly".
In a serious game, you play your best players at their best positions and you put your best available pitcher on the mound. And, you never let up.
In a friendly, you put players at different positions and have them try new stuff. You might have your #4 hitter practice bunting. You might go totally crazy and have your catcher try to steal a base. (In one friendly, I had one really athletic kid play every position on the field--it was a hoot.) You keep score, but don't pay a lot of attention to the scorebook.
In a serious game, you play your best players at their best positions and you put your best available pitcher on the mound. And, you never let up.
In a friendly, you put players at different positions and have them try new stuff. You might have your #4 hitter practice bunting. You might go totally crazy and have your catcher try to steal a base. (In one friendly, I had one really athletic kid play every position on the field--it was a hoot.) You keep score, but don't pay a lot of attention to the scorebook.