Guernsey had begun the game well, restricting France to just 59-1 off six overs.
But Kieran Le Gallez conceded 36 off the seventh over as Mukhtar Ghulami struck four sixes and a four before the French got 22 off the next over off Luke Bichard.
Oliver Clapham went for 20 off the ninth over before Josh Butler went for 25 off the final over as Guernsey conceded 103 off their final four overs.
Omer hit seven sixes in the final four overs before being caught by Alex Bushell off Josh Butler while Ghulami got seven sixes in a 17-ball 53 not out.
In reply, Guernsey lost opener Ben Fitchet early on before Tom Nightingale was caught for 17 at the start of the fifth over while Butler was caught for 11 four balls later to leave Guernsey on 35-3.
It got even worse when Mangal claimed Bichard’s wicket for 13 before dismissing Alex Bushell and Adam Martel for ducks in his next two balls as Guernsey were left on 53-6 off six overs.
French spinner Sajad Stanikzay ended with match-leading figures of 3-9 before Ollie Nightingale hit four sixes and four fours, but when he was dismissed midway through the final over Guernsey’s hopes of a point vanished.