The best diet to manage reactive hypoglycemia is a diet that keeps
insulin levels low (Because too high
insulin / overshooting of
insulin in response to carbs eaten), is what causes hypoglycemia.
So best diet will be a low carb diet, with plenty of protein (protein digests slowly and helps stable blood sugars), healthy fats (also helps slow digestion and thus less insulin requirements), and small amounts of carbs coming from non-sweetened foods / mostly non-starchy veges.