I think that T2D, a bit like metabolism in general, is one of those things where there isn't a genetic level playing-field to begin with and to criticise somebody for it and accuse them of being at fault is very foolish indeed (not to mention wholly unnecessary).
As has already been said, even people who are very slim and follow an extremely healthy diet can get T2D if they have a genetic propensity to it.
However, inasmuch as it is (or maybe isn't) linked to poor diet and/or lack of exercise, you have no way of knowing how 'easy' it would be for them to get it, or whether it would be nigh on impossible, even if they banqueted on takeaways three times a day.
Imagine if somebody told you that they couldn't afford to buy a loaf of bread, you might be sceptical of the truth of that or otherwise maybe think them terrible with money to not be able to find 55p for a cheap loaf. Now, if they told you that, soon after they were born, the midwife tucked a randomly-assigned raffle ticket into their babygro, which determined the set price at which they were allowed to buy bread for the rest of their lives, and it so happened that they got the one which dictated they would have to pay £250 per loaf if they wanted any.
All the time the topic came up, you'd be thinking "But it's 55p - or less for the value stuff - how can somebody not afford 55p for a loaf?". meanwhile, they'd be thinking "How can she possibly afford to eat so much bread when just four loaves will cost you a grand?"
Neither of you would be wrong in your thinking for yourselves, but if somebody else has been dealt an apparently random 'hand' that is at the other end of the spectrum to yours, you can't possibly understand their POV nor be in a position to judge them for how they play their hand.