M.P.
It's likely that her body has not matured enough for her to be poop free during the night.
Suggestions that have worked for other people are to put her in a larger size of diaper or to put a large size vinyl pants over the diaper.
Is her poop diarrhea? If the poop is solid a larger size diaper should keep it confined. If it's diarrhea you'll want to treat that.
Does she poop at all during the daytime? You could try putting her on the toilet a couple of times (or more) per day and see if you can train her body to poop in the daytime. I think the most likely time to be able to poop is a couple of hours after eating. I'm not sure about the time, tho.