I would imagine it would be difficult to make jam with less sugar as the sugar is what does the preserving.
The specially produced diabetic foods are full of artificial sweetners and other chemicals which give you the trots if taken too much. It is not recommended that diabetics should have these specialist foods, there is so much reduced sugar products on the market now that they are not necessary any longer.
As a health care professional myself, we recommend to our patients that any food in moderation is fine, just as sue321 says. Obviously they should not have too much sugar, but as long as it is combined with a more complex carbohydrate, like bread, then it won't be absorbed too quickly and won't result in raising the blood sugar too quickly, which is to be avoided, unless the person is actually hypoglycaemic, then some jam would be good.
Hope this helps
Rachael