Technically speaking, these two words have different meanings. Healthy means to be in a state of good health, as in the Olympic athletes are all healthy. In contrast, healthful refers to something that is good for your health, for example kale and tofu are healthful foods.
But languages change, and with respect to English the meaning of the word healthy is rapidly changing and, more often than not, healthy is used to mean healthy. Traditionalists and grammar Nannies may not like this, but there is no going back.