All I can say is as a 3rd year vet student if I had seen pictures of that dog while it was still alive I would have been obligated to speak to someone about it’s welfare. Those nails are inexcusable, no one who claims to love their dog would let them get like that, that is abuse. In regard to its weight it is certainly not on the chubby side but it’s not uncommon to see chihuahuas like that, we normally do recommend they are on the skinnier side (not underweight though) however it’s quite hard to tell from a picture about a dogs weight. What you can tell is those nails would have caused the poor thing a lot of unnecessary pain