Can Nutella be substituted for Peanut Butter in Peanut Butter Fudge?
The critical differences in a fudge recipe will be the amount of fat, nut solids, and other ingredients that might interfere with texture and setting. Since fudge gets its texture from a cooled suspension of solids in fat, and having the right concentration of sugar cooked enough to get to the correct temperature, these are the critical ingredients to consider for this type of substitution. From the nutition information:

Nutrition Facts
Peanut butter, smooth style
Amount Per 2 tbsp (32 g)
Calories 188
Total Fat 16 g



Nutrition Facts
Nutella, original
Amount Per 2 tbsp (37 g)
Calories 200
Total Fat 11 g

So, Nutella has soy lecithin as well, which can affect texture. You will notice that the fat percentage, as grams per gram of substance is considerably lower for nutella, but it has sugar, cocoa powder etc. This means that less fat = more binding, but other ingredients = less binding.
The long and short is you have to experiment. Nutella has other ingredients, too, like milk or milk solids and generally has a lighter texture than peanut butter. Experiment and you will then have to adjust other ingredients, like chocolate type or powder, butter, cream, etc... The greater sugar content is also important.