We don't know what happened to her exactly, but if she had an issue while being still deep (more than 20m) that would have been very hard to spot for a buddy, and even harder to do a rescue. She was not alone by the way.
The only way this could have been totally safe would be to dive along a line, using a lanyard to not drift away. This is the setup used in all competition or deep training and this has prevented many casualties.
Still speculative, the original article mentions a lack of spotters and an io9 article mentions a depth of 30-40m.
For her this was child's play: freediving gets exponentially harder the deeper you go. She could've decided to go exploring a bit on the bottom - at that shallow depth she'd have 2+ minutes of bottom time.
With ~5 days of training, a person can get to 30-40m. Then it becomes harder (e.g. the air in your lungs gets squeezed to a smaller volume than a complete and total exhale)
The only way this could have been totally safe would be to dive along a line, using a lanyard to not drift away. This is the setup used in all competition or deep training and this has prevented many casualties.