Demi, 49, and Martin, 37, "just kept chatting and laughing," at the barbecue, an onlooker told Life & Style.
Martin's biggest roles have come in 2002's The Ring and 2006's Smokin' Aces.
The last 12 months have been a tumultuous time for the Ghost beauty, who broke up with husband Ashton Kutcher; went to rehab for an addiction to prescription pills and an eating disorder; and clashed with daughters Rumer, Scout and Tallulah Willis.
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