"Veterinary staff had done testing which confirmed she was suffering from terminal kidney problems.
An autopsy will be done in an effort to find what led to Billie's renal failure.
Aaron Machado, from Project Dolphin Safe, says no-one wanted to see Billie dying in pain.
"Her kidneys had stopped functioning and that's going to be a very painful way to die so the decision was made under humanitarian grounds to put her out of that," he said.
"We'll wait for the toxicology reports to come back once those tissue samples are taken and then that will give us a better understanding as to what was going on internally.
"And being renal failure there's an underlying issue there and we have to find out what that is, what's caused that."
Since the source article shows Billie's name as 'Billy', I made corrections here (by changing from 'Billy' to 'Billie') so as to avoid confusion on the readers' part.