What do you tell a 9 year old child when their mom is at rehab?

long story short, my step daughters mom is going to rehab [addicted past 5 years to meth] and we are filing to take full custody (no previous custody has ever been fought)… she is nine and she will be filled with questions. What should you or do you tell a child without completely lying or ruining her mom in her eyes?

Any help
Papers are being served Tuesday and we are picking her up from school. Until hearing we will continue to take her to her school and dance class. Thanks for all the answers so far! This is difficult for us all.