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.
Of course, Grubbs shot to fame when it was revealed she was one of a bazillion women Tiger was nailin' behind his wife's back.
These days, Grubbs is still screwing around with a married man -- her brand new husband.