Decayed Voidstone obtainable without Shaper Kill

When in a party, the Decayed Voidstone can drop for players not eligible to obtain it aslong as the owner of the instance can drop the Voidstone.

Reference: 1.372.084.672

To reprocude:
Character A kills Shaper, opens Elder.
Character B does not have a Shaper kill. Joins into Elder Fight.
When the fight is completed Character B can loot a Decayed Voidstone.

This happened to me as well.