Well coached from the master helpers. Problem is solved if OP follows the hints above.
I only wanted to clarify on something from the OP what means he/she in saying:
I only want "user" to be able to see the contents of "folder".
I don't what to change "admin admin" of folder.
Do you mean the "user" group... or a particular login $USERNAME ?
If you mean the "user" group then you may need to juggle up properties of two existing groups which may not be favorable.
But if you mean "user" as particular login $USER then the solution is simple:
sudo usermod -a -G admin $USERNAME
for example: sudo usermod -a -G admin vRanger
that particular user login name as "vRanger" becomes now a true scout ranger who should be able to do what the "owner" of the folder/file can.
Hope that helps. Good luck and enjoy.