Maybe an anti-diarrhea medication taken in the evening would help. But I really think at her age it may just be a loose sphincter. If it appears to be diarrhea, then it may be leakage due to impacted stool. What happens is the impacted stool doesn't move and what she eats just goes around it, and stays in liquid form which can just leak out at any time. If she is eating a high fiber diet, plus metamucil, this could very well be the problem. Always keep in mind that too much fiber can be as bad as not enough.
You might want to make sure she isn't eating a lot of fiber at night, but that bulk of it is in the morning to midday even. Has she tried eating a banana for her last food of the day?