As I see it, your developer wants to be sure he gets a full table scan when he selects a value that returns many rows.

I did a little test. I hope the results can help you.
btw, i'm on 7.3.4

Create table gdn ( nummer number(4), nummer number(4),tekst varchar2(20));

create index gdn_i2 on gdn ( nummer,nummer2)

I inserted records in the table

nummer = 1 , nummer2 = 1, tekst = 'ONE'
about 16000 records
nummer = 1 , nummer2 = 2, tekst = '12'
about 7 records
nummer = 2 , nummer2 = 2, tekst = '2'
about 4 records

analyze table gdn compute statistics;
analyze index gdn_i2 compute statistics;
analyze table gdn compute statistics for columns nummer,nummer2;

select count(1) from gdn where nummer=1;
-- results in full table scan
select count(1) from gdn where nummer=1 and nummer2=1;
-- results in full table scan
select count(1) from gdn where nummer=2;
-- results in index scan using GDN_I2
select count(1) from gdn where nummer=2 and nummer2=2
-- results in index scan using GDN_I2

If this is the result you developer wants to achieve,
the index 2 is not necessary.

Hope this helps
Gert