RobinW, I'd second much of what Babynick has written, so I won't repeat it.
Especially agree on the bath issue.A no-no unless the children are absoultely clamouring for one and it's seen as a fun and splashy way to end the evening.
I'd also definitely pre-arrange an early morning pick up. In my experience it's in the morning that any trouble begins, with children getting on each other's nerves. It can be horrible!!
My son has had different friends, boys and girls, coming for sleepovers since he was about 5. We only have one friend at a time, so it's manageable. It's usually pretty enjoyable for all concerned, too. In the evening, the children are usually well behaved, because they see a sleepover as being such a treat - and there's always that possibility that you might cancel it if they get really out of hand. A threat to keep up your sleeve. We've never had to use this, though.
I insist on my husband being there for the evening as well, for practical and moral support. No slinking off for a few hours. Also, lay on some surprises and distractions, like Kinder Eggs, balloons, etc to diffuse any difficult moments.
In my experience, the children always fall asleep within an hour of bedtime. To encourage this we make bedtime an hour or (even) two hours later than normal. Watching a video pre-bed also helps to wind things down.
Lastly I always check with the child's parents about their sleeping/eating habits - the usual stuff. The worst combination is having one child who is a late-nighter with another who wakes up at the crack of dawn. Avoid!!
But on the whole I'd say yes, go for it. It should make your dd very happy along with her lucky friend.
PS Babynick: might need to pick yor brains on Beaver matters some other time, if that's Ok with you.