People with diabetes used to be advised to count carbohydrate exchanges or portions. Research has shown that it is not essential to measure the carbohydrate intake in order to achieve good blood glucose control. Eating starchy foods   regularly and making them the basis of all meals is more important. Recommending portion sizes somehow gives the idea of restriction, when in fact it is fatty food and protein foods which are now more important to regulate in terms of portion size. I have diabetes.
Bananas are a high fibre fruit and can be included in the diet of someone with diabetes.
Try to keep to one banana at a time. Indeed, you should spread your intake of all fruit over the day. Grapes can be eaten in a normal portion size of about 8-10. The problem is that it is easy to eat a lot more than this without realising. If eaten in large quantities, there may be an adverse effect on your blood glucose level.