Honestly a tiny piece monthly of fruit is most likely not going to hurt a diabetes prone hamster.If it were a particularly sugary fruit,perhaps it could be problematic,but not all fruit is extremely high in sugar so it really must be considered on a more individual basis rather than an outright assumption that every single fruit is equally bad
It should be possible to find exact values including amounts of sugar for certain portions of fruit though nutrition websites,preferably government run ones,and with that information it would be a lot easier to determine within the variety of safe fruit what would be best if you really do want a special occasion treat of fruit.