It's quite common for an instructor to dive alone with students. Even a first level freediver knows how to save someone from a shallow blackout; it's unclear if her diving partners were suitable or if she took excessive risks. This is a good reminder that no one is invincible.
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.
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)
http://io9.com/well-probably-never-know-what-happened-to-fre...