Is my cat dieing before my eyes?

my cat is a curious case, he is the neighbors cat but my dad believes they don't really like him. So my dad took him in and started feeding him twice a day, but he still goes to the neighbors house and lives there too. My sister and I believe that on top of our feedings that the neighbors are feeding him twice a day too. He is getting rapidly fat and I am scared he will die of a heart attack or something but my dad doesn't listen. Do you think my cat is in jeopardy of dieing? PLEASE HELP IF YOU CAN! Thank you :)