Oh dear. I'm afraid you may not like my opinion. I have had cats my whole life (more years than I'm going to admit! LOL) Here is my opinion: Cats are domestic animals just like dogs. They are NOT wild animals. Just as you wouldn't let your dog run loose, you should not let your cat run loose. And one of the main reasons is exactly what happened to your mom's cat, among a host of others. According to almost all animal experts I've ever read, and every vet I've had a relationship with, cats should be indoor animals only. If they are allowed outside, they should be supervised, or in an enclosure they cannot get out of. While cats are incredibly smart (for an animal), they cannot reason any higher or better than a toddler. And you would not allow a toddler to roam unsupervised, would you? There are people who say "Well, he WANTS to go outside. He LIKES it!" And my reply is "Well, your toddler WANTS to run out into the street without looking. Or your teenager WANTS to experiment with drugs/alcohol/sex. So what? Who is the Human Owner (Parent) and who is the Animal (Child).
I am very, very sorry this happened, but I believe that the full responsibility lies with the owner of the cat. And I do have both dogs and cats.
I hope the kitty makes a full recovery, and stays in the house for the rest of his natural life.
PS My opinion would be the same even if it were a strictly rural/farm area.