Step 1: Understand the facts.
A decree is passed against several defendants. One of them never got summons and got no chance to defend. We must find his proper remedy under the Code of Civil Procedure, 1908.
Step 2: Recall the basic fairness rule.
A core rule of natural justice is that no one should be condemned unheard. The CPC makes sure every defendant gets notice and a chance to defend.
Step 3: See why summons matters.
Summons tells a defendant that a suit has been filed, what the claims are, and when to appear. Without proper summons, he cannot defend at all.
Step 4: Apply Order IX Rule 13 CPC.
When a decree is passed against a defendant who did not appear (an ex-parte decree), he may ask the same court to set it aside if summons were not duly served or he was kept away by sufficient cause. Non-service of summons is a strong ground.
Step 5: Check the other options.
Review is not the proper remedy here. A separate suit is usually barred when a direct remedy exists. An appeal may be possible in some cases, but the direct statutory remedy is the application to set aside the ex-parte decree.
Step 6: Reach the answer.
Since this defendant was not served, his proper step is to apply to set aside the ex-parte decree under Order IX Rule 13.
\[ \boxed{\text{By applying for setting aside the ex-parte decree.}} \]