They've been "friends for years," a source told Us Weekly .
Culkin's presence at the intimate ceremony comes on the heels of a National Enquirer story alleging that the former actor is a heroin addict.
His rep responded to the blockbuster story, calling it "not only categorically without merit, but it is also impossibly and ridiculously fictitious."
Culkin's super-thin appearance has had fans concerned about the Home Alone star for months though.
Macaulay had an eight-year long romance with Portman's Black Swan costar Mila Kunis that ended in late 2010. He has only rarely been seen in public since their break-up.