Davids, 39, joined League Two's bottom side this week to assist manager Mark Robson and was quickly given a taste of their troubles when Rhys Griffiths converted a Plymouth penalty after just two minutes.
Maxime Blanchard headed the Pilgrims two-nil up on 11 minutes and while Dave Stephens pulled one back before the break, Paris Cowan-Hall and Luke Young eased Argyle home.
The Bees have failed to win a single game this season and fell behind when Young was brought down in the penalty area, leaving Griffiths to fire home from the spot. Blanchard was then allowed the freedom of the Barnet box to send a rasping header into the top corner from 12 yards.
Former Fulham and Holland striker Collins John limped off after 15 minutes of his Bees debut but Stephens rifled home to give the hosts hope.
Barnet continued to push forward in search of an equaliser after the break but it was Plymouth who wrapped up the win courtesy of Cowan-Hall's 62nd-minute header and Young's stunning free-kick in stoppage time.