All facts need to be known to understand if it can be challenged. Was it contested by both parties? Which law was applied? Was the law significantly different than the applicable law in India? (Also U.S. marriage/divorce laws vary across states). Were both parties given fair opportunity to contest or participate? How much time has passed since the U.S. divorce decree? Did any of the parties remarry thereafter and if so, did the opposite party raise a timely objection? What was the domicile of the parties at the time the U.S. divorce decree was passed? Were domiciles the same? Domicile is not a simple issue of presence for a short time....
And so on...
A good advocate has enough prima facie grounds to challenge it in India.... Whether he will win is a different issue.
Disclaimer: I am not a advocate.