It kinda depends on how much time you have ave fo play.
In general, the fast acting oral laxatives will make her have a very sloppy, messy poo. The slower acting laxatives generally cause someone to have a much more solid, formed poo.
Fast acting laxatives are the salines, like Milk of Magnesia, Phospho-soda, Glauber's Salt, or Epsom Salt any of which usually produce a very urgent, sloppy poo within about an hour.
The other class of fast acting laxatives are the non-digestible sugars, like Lactulose and Sobitol. These usually work within a couple of hours and results are often preceded by a lot of gas so the impending poo is heralded by a lot of trumpeting. These also tend to produce loose poo, although not as lose as the salines.
If a person is given either a saline or sugar type laxative, a good protective panty over the diaper is strongly advised. A person can pass a lot of sloppy poo in a very short time..
If you have more time and want a more controlled poo, I'd suggest a dose of Senna. This is generally taken at bedtime and lets the recipient sleep though the night without interruption. A full dose usually starts to work around breakfast time but it can take longer. The recipient usually poos by lunch time. If senna is taken without a stool softener, like Colace or Mialax, the poo is generally not soft, but is firm and formed.