The former Holland midfielder is the new joint head coach of the Football League's bottom club, his first managerial position.
But there was no fairytale start for the former Champions League and three-time Serie A winner.
His new side fell behind after just two minutes as Rhys Griffiths converted a penalty and Maxime Blanchard made it two after 11 minutes.
David Stephens pulled one back five minutes later, but Argyle ran away with it in the second half as further goals from Paris Cowan-Hall (62) and Luke Young (90) earned them a convincing win.
Defeat left Barnet seven points from safety and five behind second-bottom Aldershot, who were thumped 4-0 at leaders Gillingham.
Charlie Allen opened the scoring after 11 minutes and Danny Kedwell added another from the spot (27).
Myles Weston made it 3-0 by half-time and Chris Whelpdale completed the rout midway through the second half.
Cheltenham knocked Fleetwood out of the automatic promotion places by moving up to third with a 2-1 win at AFC Wimbledon.
Marlon Pack put the visitors in front after 25 minutes and Kaid Mohamed doubled the lead early in the second half.
Byron Harrison pulled one back in the 90th minute, but it was too little too late.
Fleetwood dropped to fourth as they were beaten 1-0 at home by struggling Wycombe.
Bruno Andrade scored the only goal of the game in the ninth minute.
The three other sides in the play-off places, Burton, Rochdale and Exeter, all failed to win.
Burton were held 1-1 at home by Bristol Rovers, Billy Kee's 50th-minute equaliser for the hosts cancelling out David Clarkson's opener on the half-hour mark.
Rochdale lost 2-1 at home to Morecambe, for whom Jack Redshaw scored twice (10 and 66).
George Donnelly gave Rochdale hope by netting in the 73rd minute, but they could not get a leveller.
Exeter went down 3-0 at Northampton, whose goals came from Adebayo Akinfenwa (25), Kelvin Langmead (39) and Alex Nicholls (84).
The Yorkshire derby between Bradford and York ended in a 1-1 draw at Valley Parade as Zavon Hines' 59th-minute leveller for the hosts cancelled out Ashley Chambers' goal just before half-time.
Elsewhere, Britt Assombalonga scored twice as Southend stuck three times after the break to claim a 3-0 win at Rotherham. David Martin completed the scoring.
Torquay were 3-1 winners at home to Accrington.
Rene Howe put United ahead with a fifth-minute penalty, but Padraig Amond levelled in the 65th minute.
Billy Bodin put Torquay back in front two minutes later and Lee Mansell made the points safe 10 points from time.
Dagenham won 2-1 at Chesterfield thanks to a brace from Sam Williams either side of a Jack Lester leveller.