Res judicata is not applicable for fraud. Fraud vitiates all acts. This can be challenged in any court lower or higher or in any proceeding or collateral .Defendant's advocate files his objections on res judicata when the issues are to be decided by facts of issue and law, the second suit can be filed as declaration suit. The details provided in this post is ambiguous and not complete.
What is the first case ? What issues were formed and what was decided in the earlier case?
What is the present case ? What reliefs/prayers are made in the second suit?
What specific issue brings res judicata into the picture?
Contact a local advocate as the issue appears complicated and entire proceedings have o be gone through before deciding on the second suit.
If the first suit is of partition and dismissed, and if the second suit is for declaration and cancellation on fraud, it may not be a res judicata.