In a medium saucepan over medium-high heat, bring 4 cups water to a boil. Add bouillon, stirring until dissolved. Add chicken; cook until cooked-thorough, about 20 minutes; transfer to refrigerator until cold. Shred chicken. In a medium bowl, stir together chicken, mayonnaise, onion, guacamole, garlic and cilantro. Season with adobo. Gently fold in avocado. Refrigerate until ready to use.
In medium bowl, stir together masarepa and salt. Add 2 ½ cups lukewarm water. Stir until combined; let sit 5 minutes. Using wet hand, knead dough until smooth. Divide dough into 8 portions. Form dough into small balls.
Heat oil in large comal or non-stick skillet over medium-high heat. Add balls, leaving at least 3" space between dough. Using palm of hand, flatten balls to ½” thickness. Cook, flipping once, until golden brown on both sides, about 7 minutes total. Repeat with remaining dough.
Using knife, halve arepas horizontally without detaching end. Divide filling evenly among arepas.