I would also encourage her to drink lots more water. This is very important for soft poo and will help the lactulose do it's job.
Have a star chart for this perhaps, rather than one for pooing in the loo, at the moment. Does she drink much at school?
I wouldn't worry about getting her to go in the loo for the moment. Just concentrate on getting a daily soft, painless poo, even if it means a nappy. Lots of praise whenever she does one, no telling her ''poo, it smells'' or anything negative .
Once you have accomplished this, insist she goes into the bathroom to poo- even if she just stands in there. She'll hopefully get the idea that is where she is expected to be. Always pour the poo down the loo from the nappy. Get her to flush, lots of praise.
Some posters have a leaflet/book called ''Poo goes to Pooland'' or something similar. Maybe do a search and see if you cna get a copy emailed to you. I think it's about Poo going down the loo to join his friends in Pooland, or something .
Next step would be for her to sit on the loo in her nappy. I think some children find it hard/odd to sit down to poo when tey are used to doing it 'on the run' for years .
Then sit on a nappy on the loo, etc....
Keep up the praise with each step.
If you use a sticker chart, make sure they are achieveable steps so she does not get disillusioned.
Sorry, a bit garbled but hope some of it helps.