Cant get my searing exarch voidstone

I helped my friend completing the searing exarch. We were both in the map when it died and it dropped 1 voidstone which was my friends. Becouse of that i cant have my own searing exarch voidstone. I have tried doing the 28 +14 tier maps in influence of searing exarch but even when i kill the exarch again i cant get the voidstone. Please help.

I did he dont have it.