In Digambar Jain tradition, Mallinath is not accepted as a female Tirthankar. They hold that:

1. A Tirthankar must be born in a male body.
2. A female body cannot attain moksha in that tradition.
3. Therefore, if a text or tradition describes Mallinath as female, Digambar scholars treat that as not authoritative for their doctrine.

So their position is not “how to prove a female Tirthankar,” but rather:

• They do not accept Mallinath as female.
• They identify Mallinath as male.
• The Shwetambar view that Mallinath was female is rejected by Digambar tradition.

Why this difference exists

The difference comes from a sectarian doctrinal difference:

• Shwetambar tradition: accepts Mallinath as a female Tirthankar.
• Digambar tradition: accepts only male Tirthankars, so Mallinath is understood as male.

In simple terms

For Digambar Jainism, the answer is:

A female cannot be a Tirthankar, so Mallinath was not female according to Digambar belief.

This is a difference in tradition, not a matter of one side “proving” the other wrong in a universal sense.