My personal opinion is that you tell the truth but omit the gory details. Mommy is sick and going to a place that can help her with her problems. Even at 6 I think we underestimate what they have seen and heard on their own. Sometimes it can be even more confusing for them. If they have seen mommy acting "funny" or seen the other bad effects of drugs they may think that is normal behavor for a parent and that is unhealthy for the child. Truth is always best but a small child doesnt need too many details that would just confuse and scare them. I think you may find that this child knows much more than you realize and that could be a good place to start. Gently talk to her and see if there are things she will open up about that bother her or she has seen and then explain to her that the place her mommy is going will be helping her with those problems. At this age many times its better to have these talks while you are doing something else - playing a game, baking cookies - just a casual talk while you are doing something. Then the things that she brings up that bother her - even if its the fact that mommy isnt around much, you can explain are the things this place will try to help with. Of course, never talk badly of her mother to her - that can have devestating effects on a child who unconditionally loves their mother.
The effect of your honesty will also mean that the child will trust you more in the future. At some point in her life she will put 2 and 2 together and its better for you to explain it then for her to try to make sense of it on her own. I wish you the best with this and thank you for caring for this innocent child during this time.