2 artists using same Garage Band loop-copyright violation?

SCENARIO

Party A is accusing and has reported Party B of copyright violation when the only thing similar from their song is the Garage Band piano loop.

Is there any copyright violation or owners rights going on here? Who is right, who is wrong?

PARTY A
This party has used about 16 bars of a piano loop from Garage Band, tacked onto the end of their song. They have commercially produced and are selling this song.

PARTY B

This party has used various piano loops and drum loops from Garage Band as the entire composition for background music for their Photography video on YouTube. They are not selling anything, and feel that PARTY A cannot site a third party violation accusation upon them because Party A does not exclusively own the piano loop anymore than anyone else who uses Garage Band.