You'll need a separate account to grant the read-only access to. I would suggest adding a role that you grant read-only access to as well-- you can then re-use that role if more users need this access in the future.
CREATE ROLE my_read_only_role;
FOR x IN (SELECT table_name FROM dba_tables WHERE owner=<<schema name>>)
EXECUTE IMMEDIATE 'GRANT SELECT ON ' || x.table_name || ' TO my_read_only_role';
FOR x IN (SELECT table_name FROM dba_views WHERE owner=<<schema name>>)
EXECUTE IMMEDIATE 'GRANT SELECT ON ' || x.view_name || ' TO my_read_only_role';
GRANT my_read_only_role TO new_customer_account;
Once that is done, the new account will need to prefix the table names with the schema name to select the data. Alternatively, you could create public synonyms for each object (you can add another EXECUTE IMMEDIATE to each loop in the code above). Or you could have the user run the command
ALTER SESSION SET current_schema = <<schema name>>
on login. You could also create a login trigger in the new account that would do this automatically. That will cause
<<schema name>> to be implicitly added as the schema prefix. It does not affect the privileges of the session-- the user still has the read-only privileges, the default schema name has just been changed.