So one report says the DM knew she surfaced, but the boat didn't see her and kept following the group. The other says nobody noticed she left the group. Whatever the case, she ascended solo not that long into the dive.
I'm sure we've all see the scenario, or maybe even done it ourselves. Diving with a group and one person is low on air or has an issue and decides to go up. The buddy (if they have one) and/or DM watches them as they go up until they break the surface. Some DMs, depending on what point in the dive, will shoot their marker so the boat knows a diver is coming up. Others don't. Some divers have their own SMBs, some don't. Some have them and have no idea how to deploy them. I think that group diving, especially if you're in a small group and are not traveling with a dive buddy, it's easy to have this happen. I know divers who choose Cozumel because as a solo traveling diver they like the group aspect of the diving.
This isn't the first accident where a diver ascended solo. DMs can't see everything all the time. Divers need to take some responsibility too, both for themselves and their fellow divers in the group. If you see someone getting ready to go up alone, get the DMs attention if they haven't, or maybe even consider buddying up and going up with them, even if it does cut your dive. If you're the one that wants to ascend, get your DMs attention and also get someone to go up with you. Carry an SMB, even if it's a small one, and know how to use it. Having a whistle or other device is good too. If the boat doesn't see you in a few minutes you might need to get their attention. I'd rather apologize for being a little obnoxious then not be seen.