According to the references:

1 medium aprox 4 inch dia grapefruit contains 20.68 grams of carbs.

For that you could be eating a ton of salad and fresh veggies loaded with micro nutrients and vitamins.

I would say nope to the grapefruit. Go with something that has more bang for the buck

I am curious as to whom told you it was the best at burning fat.

As far as the eschewing of fruit.....I eat berries and cantelope. But in moderation of course and not every meal