It is 'Akkusativ'!
The article of 'Nachricht' is 'die'
So in accusative you use 'deinE Nachricht'
In Dative it would be 'deiner Nachricht'
The article 'die' remains 'die' in accusative and becomes 'der' in dative,hence with pronouns or indefinite articles it has 'E' in accusative and 'ER ' in dative as ending.
DiE Nachricht ----- DeinE Nachricht ( Nominative)
DiE Nachricht----- DeinE Nachricht ( Akkusative)
DER Nachricht ----- DeinER Nachricht ( Dative)
Your German friend is right!
Hope it is clear :)