None of us should ever be guilty of allowing one of our dive buddies to place themselves into this situation. As we all can see, the stakes are just way too high...
That's easier said than done. One of the things that I think might have played a role in the accident is that since Marcia was such an experienced diver the others didn't concern themselves too much with her gear config, even though it was new to her. There was an inherent trust (and not completely unfounded) that she knew what she was doing. The day before the dive she was tweaking the configuration and even consulted someone from a shop about it. To all involved it looked like she had things under control and nobody had any inclination that they were "placing her" into any kind of "situation".
Hindsight is always 20/20 and it's easy to say after the fact that they should have looked at her config. If they had known then what they know now then they would have.... but suggesting that they "allowed" her to put herself in danger suggests that it was a conscious decision, which it was not. There was an assumption made about her knowledge of the configuration. It's not like they knew it was dangerous and say, "whatever".
The obvious lesson to take from this is to be aware that when someone is making major changes to a configuration that as a buddy it's wise to get involved in that process. It wasn't done in this case but it could have made a difference.
R..