Tuna is very high in protein and certain amino acids such as leucine (most essential BCAA for protein synthesis, i.e to build muscle), vitamin b-12 and a decent amount of potassium. Avacados are high in healthy fats, slow burning carbohydrates and a bunch of other minerals and vitamins.
So if you were to switch these two for a protein shake, you would be sacrificing some healthy fats and healthy carbohydrates for a little more protein. This really depends on your goals, if you're trying to build muscle, I wouldn't make the substitution since your effectively sacrificing calories as well. If you are trying to lose weight, this wouldn't be such a bad idea.
It is important to remember though that whey protein supplements are exactly that, supplements. They should supplement what you are deficient in that you can not get through your regular diet.