I’m so sorry your sister lost her baby lovebird. Sadly, it is highly likely that Grebo died from ingesting the onion and garlic. Unfortunately both are very toxic to pet birds, particularly in raw form, and being such a small bird, if he did ingest even a small amount of either or both, it could have set off a toxic reaction that resulted in such a sudden death. Another possibility is a bacterial or yeast infection related to being hand fed. It’s easy to contaminate hand feeding formula, and even the most experienced hand feeders will end up with a sick chick from time to time. However, for an older baby, a sudden death from an infection is not as common. Birds will try to hide signs of illness, but there is generally still time to get a bird to a Vet when you realize it is sick – depending on the cause. He most likely would have been refusing his formula if he had a crop or intestinal infection. Of course this is all speculation. He could have had something wrong with him from the start such as a genetic defect or a virus passed on to him from one of his parents while he was still in the egg. Only a vet could try to determine the actual cause by running tests.
I’m sure your sister will not want another bird right away. But if she does decide to get another, please urge her to read more about pet birds and particularly read about toxins. There are a lot of foods that can be toxic, but there are even more airborne toxins. Birds have such a sensitive respiratory system, they can die from being exposed to almost any strong fume, even if it isn’t something that affects people. Birds are great pets, and I hope this experience doesn’t keep her from getting another one in the future. But I would advise a hand fed bird that has already been weaned to a good diet. Handfeeding isn’t as easy as it sounds and weaning a bird takes a lot of time and patience to teach him to eat the right foods.