hi tinypoodle,
man su (debian)
-, -l, --login
Provide an environment similar to what the user would expect had
the user logged in directly.
When - is used, it must be specified as the last su option. The
other forms (-l and --login) do not have this restriction.
i tried your suggested line:
su -c '/usr/local/bin/script' $USER
and my variation without the option -l :
su -c $USER '/usr/local/bin/script'
=> both didn't succeed

sudo -u $USER -c '/usr/local/bin/script' doesn't work either.
thank you for your help.