The Elizabeth College pair both hit half-centuries in their second consecutive century stand as the home side chased down 173 at the College Field to win by eight wickets with 13 overs to spare.
Ridgwick made 56 from 77 balls including eight fours while Birch finished unbeaten on 90 from 104 deliveries with 11 fours and a six.
Earlier, Charlie Forshaw had been the pick of the Guernsey bowlers as Italy were bowled out for 172 inside 45 overs.
The young seamer had made the initial breakthrough when he had opener Risal Hettiarachchilage caught by the captain Ollie Clapham in the covers and Forshaw returned later to mop up the tail with two wickets off successive balls to finish with 3 for 14.
There were also two wickets apiece for Clapham and Birch as well as one each for Louis Steyn and the Thomas brothers, Marcus and Dan.
Opener Gianluca Longo top-scored for Italy with 44.
The result means Guernsey will face the Group B runners-up Norway in today’s semi-final at the KGV while Italy return to the College Field to take on the Group B winners Isle of Man in the other semi-final. Games start at 11am.