FROM TMZ....
Brittany Kerr's friends may have been rubbing off on her ... 'cause before she hooked up with a very married country singer, Kerr was partying with a girl who knows a thing or two about home-wreckin' ... Tiger Woods mistress Jamie Grubbs.
TMZ obtained pics of the former "American Idol" contestant ragin' with Grubbs at a bar earlier this year -- and sources connected to the ladies tell us they've been friends for a while.
We're told the two initially met while working together at an L.A. bar ... and hit it off.
