Brighton & Hove Albion defender watches Shoreham defeat
Republic of Ireland international Duffy, 26, watched close friend and Mussels' Sean Roddy as Shoreham lost 4-1 to Whyteleafe at Middle Road.
Ryan Stevens' first Mussels goal had given Sammy Donnelly's already-relegated side a surprise half-time lead but Whyteleafe stormed back.
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Hide AdGoals after the restart from Bola Nkori-Venceslas, Scott Day, Tom Pearson and Joey Taylor took the visitors to victory.
Shoreham boss Donnelly said: "Once again individual errors cost us. We played really well first half but conceded so soon after the break and never recovered.
"This league is unforgiving and it's been a really long, hard season."
Shoreham's final league game of the season is at Phoenix Sports on Saturday.