Answer

Generally, no — we should not casually say that Nilanjana did santara (sallekhana) unless a specific Jain source clearly says so. In most Jain tellings, she is described as disappearing / leaving / dying when her allotted lifespan ended, not as voluntarily undertaking sallekhana.

As for Indradev, in Jain tradition he is a deva, and devas can have avadhi jñāna by birth. So if a story says he knew her lifespan was ending, the simplest Jain explanation is that he knew through his divine knowledge. If someone says “without avadhi jñāna,” that would not fit the usual Jain understanding of Indra.

Explanation

In Jain Dharma:

• Sallekhana / santara is a voluntary, disciplined spiritual vow taken with detachment, usually at the end of life.
• It is not the same as ordinary death.
• So unless a text explicitly presents Nilanjana as consciously observing sallekhana, it is better to say:

- she completed her life-span - or she departed according to her karma

About Indradev:

• Devas are said to possess divine cognition.
• So Indra would not need human guesswork to know that someone’s life was nearing completion.
• In Jain thought, life-span (āyu karma) is fixed, and beings with higher knowledge can perceive such things.

Spiritual Understanding

This distinction matters because Jain Dharma is very precise:

• Natural death and sallekhana are not identical.
• Karma-based lifespan is different from voluntary spiritual fasting at life’s end.
• Divine beings like Indra are not portrayed as ordinary observers; they can know through their special knowledge.

So, from a Jain perspective, it is better to keep the story doctrinally accurate rather than using santara loosely.

Takeaway

• Nilanjana usually should not be described as doing santara unless your specific source says so.
• Indradev likely knew through avadhi jñāna, because that is part of the traditional Jain understanding of devas.
• If you want, I can also explain this in the context of the specific Nilanjana story from Jain tradition.