R.R.
First off, when someone leaves their child with you and doesn't come back for them or make arrangements for them to be picked up at a designated time it is considered parental abandonment, contact your local PD if this ever happens again, they will take a child or children into protective custody and then the parents get to sort it out with social services. Perhaps such an intervention is exactly what Susie's parents need to help them get their priorities straight.
Secondly, I would be so proud of my daughter if she said she would rather have a friend for a short time than not at all, I believe she really understands that friendships don't always stay the same. Not that I would want her to ever be hurt again (which sadly she will be at some point,) but she has thought it through, and being willing to take a risk shows so much maturity for her almost 10 years!
Thirdly, go easy on Susie, she is a product of the dysfunctional chaos in her life. It doesn't mean you excuse her behavior, but you know why she is the way she is.
The tween and teen years are filled with so much drama, you can't get involved every time, your job is to build up your child and watch her blossom as she learns from her experiences Take a step back from the situation, encourage her maturity and tell your daughter to decide what she is comfortable with and to go with that, and that you will support her decision whatever it is. I hope she has a great 10th birthday!