How do I protect my granddaughter without enabling her mother?

My 25 year old daughter has difficulty with responsibility, and continues
to make poor decisions. In the midst of this irresponsibility is my 2 1/2 year old grand-daughter.

My daughter seemed to stop developing mature decision making skills following the sudden death of a close friend when she was 13. Her emotional and psychological development seemed to hit a wall when she was 16. Her level of responsibility seems to be stuck in "teenage" mode where she is rebellious, is very self focused, and makes poor decisions.

Due to several upheavals, she and her 2 1/2 year old daughter now live with my husband and I,
but she continues to make poor decisions - eg. she is a very social person and has a "need" to
hang-out with friends, etc. even though her daughter needs to spend quality time with her, even if it's just knowing that mommy is home.

Whenever I point out that she needs to think about
her daughter's emotional needs more, she becomes defensive and finds any way to justify why
she needs to visit friends, etc. If I put up arguments as to why she should stay home, or that
I am busy or won't babysit, she says she'll drop her daughter off at her dad's or his parents -
even if that could entail driving in poor weather etc. to do that, or late at night, neither
of which is in her daughter's best interest - just so that she can do her thing. She figures that she's taken care of her daughter's needs and so that makes everything OK.

So my dilemma is how do I not enable her to continue poor decision making while at the same
time making sure that my grand-daughter isn't being bounced around like a piece of property
at the convenience of her mother so that she can do her own thing?

Thanks for any advice on this. I understand that there are many intricacies not described
here in this short summary, but the same problem continues throughout all of them.