no, you dont have to takee care of it. Its her responsibility, and she probably leaves it to die because shes bored of it. The tama isnt really "suffering" because its not an actual lifeform so theirs not really no harm done even thought you are supposed to take wise care of it. If you want to take care of it and feel that it is wrong ask her why isnt she playng with it and if shes bored of it ask if you can have it. I know alot of kids that give their tamas to their Older brother or sister because their bored of it. Its not your fault that she is lacking in her responsibility of taking care of her tamagotchi. then again, this is my oppinion.
Yeah, it's her job to care for it, but I really hate to see my brother's continually die and the battery go to waste because he won't play it, so I care for it. Besides, I need his to connect with mine!