Maria, my deepest condolences on your loss.
While I do believe the Aleve tablet that she ingested played a part in her passing, it was probably not the only factor given her lethargy/laziness and refusal to eat on and off the last few months of her life.
Aleve contains naporoxen, which is a nonsteroidal anti-inflammatory that we do not use in dogs due to the high incidence of side effects including gastrointestinal bleeding, ulcers (sometimes perforating) and kidney damage even when given appropriate doses.
The naproxen dose listed in Plumb's Veterinary Drug handbook is 2mg per kilogram (2.2 pounds) of body weight every 48 hours.
In a dog her size (55 pounds/2.2 lbs per kg = 25kg) a usual dose would be 50mg, so the dose she ingested was roughly 4 to 4 & 1/2 times a normal dose if she was given one 220mg tablet.
A single (meaning given all at once) toxic dose of Aleve in dogs is 35mg/kg, or 875mg or roughly 4 tablets of the 220mg size for a 55 pound dog.
Peak blood levels are within 3 hours of ingestion so these levels should have occurred the first night, but could have been as much as a day later in a dog with already compromised kidney or liver function and would have explained her vomiting a couple days after that first dose.
Toxic side effects are abdominal pain, vomiting, gastrointestinal ulcers which can perforate, dark tarry stools, weakness, anemia, and kidney damage shown by increased kidney waste product levels (BUN and creatinine) on blood testing.
Unfortunately the half life (the amount of drug metabolized and decreased by half) in dogs is very long, 72 hours, and as such the effects of the drug linger.
In her case with her history of lethargy & a poor appetite I suspect that she had old age changes to her kidneys and/or liver such that their function was not normal, and naproxen further pushed them into complete failure. Kidney failure predisposes to gastrointestinal irritation and ulcers forming. If we add a toxic dose of naproxen the ulcers could perforate leading to a rapid decline in health and death.
In short I suspect that the combination of aging organs and a very toxic drug led to a quick decompensation and death.
I understand how much you must miss her, and how heartbreaking this was for you, but I hope my explanation leads to an understanding and acceptance of what happened, and gives you some peace.
I think your girl wasn't well to begin with unfortunately.
Please let me know if I can explain any of this further.