The popstar is convinced Ghalib has cheated on her and is adamant their relationship is over, according to The Sun.
Spears reportedly confronted the British paparazzo after two video clips of him spending time with mystery women were posted online.
She then checked his iPhone and discovered a series of "sexy" messages all from one girl.
"There were about a dozen from one girl, all sent on one day," said a source. "They were pretty saucy stuff with sexual references - certainly not the sort you’d send to just a friend.
"Britney lost it and started yelling. She was demanding to know who sent the texts and shouting, 'What’s this about? You’re cheating on me'.
"Adnan said the girl was just a friend, but Britney got more and more angry. Then she told him, 'That’s it. It’s over'.
"Just before she told Adnan to go, she took the phone and threw it in the pool right in front of him. He didn’t even bother trying to get it out of the water."