GERARD GREEN OVER JEN MAKING MUSIC WITH JOHN MAYER

Jealousy triggered a nasty battle between JENNIFER ANISTON and GERARD BUTLER at a swanky Cabo San Lucas resort, where Jen celebrated Birthday 41 with her Scottish swain, and BFFs COURTENEY COX and SHERYL CROW!

Trouble flared, sources say, when careless Cox suddenly asked Jen: "How are your singing lessons with JOHN MAYER going?" - then bit her wagging tongue when Gerard, just back from a swim, reacted like he'd been gut-shot!

Staring bullets at Jen, Gerard growled: "Yes…how are those voice lessons going?" Then he stormed off to the bungalow, while distraught Jen and her grrrls brainstormed a defense for her outed secret: Ex-loverboy Mayer's been giving her private singing and guitar lessons in prep for her next movie, "Goree Girls" - about female convicts performing behind bars in a country-western band.

Said an insider: "Jennifer spent hours alone with Gerard, trying to convince him there's nothing between her and John - that he's just a friend helping her out."

Butler wasn't buying it at first, but finally the couple emerged arm-in- arm and smiling…as mortified tattletale Courteney sighed in relief.