What excuse IS THERE for consistently standing in fire in a raid? OK, sure, when you're learning the fight and there's a lot of stuff going on... and there's the occasional (rare) lag spike...but after you've done a few attempts people should know WTF is happening and react well.
I hate to say this... ok no I don't... but NOW is the time to sit people like this. Don't gear them, drag them along and then finally bench them in T15. Get other people in there now who can play decently and not fail at the most basic of tasks.