My advice would be to just cut the girl some slack. She's only 8. On some level, she probably knows what her mom is, but that's such a huge thing for a child to admit - that their parent is sorely lacking. So she says things to make her mom look better, to make herself feel better...I just have a lot of sympathy for this kid.
I also don't think that she has much of an idea of how her mom is at this point. It has only been in recent months that she has really been cognizant of things going on (asking questions about "adult" matters). Every previous break-up has been handled in the same way: DSD's mom starts dating a new guy. No explanation. It took DSD six months to figure out that they weren't moving to Greece because her mom refused to tell her. If/when this one ends, she'll know because they will have to find a new place to live and she will lose all of her friends. I really hope that her mom finds the man/house of her dreams soon, because now things are starting to count - DSD is more emotionally invested.
They still haven't explained why DSD's sister (who lives with her at her mom's house) has been in and out of the hospital for the past few months and will not be returning to school. I realize that she doesn't need a lot of details, but she needs to be told something. Especially since she is starting to pick up on things/adult conversations.
One mistake that I realized that I made is that DSD is 7 (going into 3rd grade), not 8! We've been talking about planning her 8th birthday party (early fall) and DH is always referring to her as "nearly 8," so the "8" got stuck in my head. I better go modify my signature.