I don't know about nursing moms but when my hedgehog had funny poop I always gave him a bit of butternut squash baby food. Plain pumpkin pureé works the same. If she won't eat it on her own I don't know what to do. My hedgehog loved it but would only eat soft food from a syringe.
How long has she had the diarrhea? Make sure she's drinking plenty of water, since diarrhea can quickly dehydrate a hedgehog, especially if she's nursing on top of that. Squash/pumpkin can help firm up poops, but I would also try and get a sample of it to take into the vet's for a fecal test, so it can be checked for any infections or anything that might be able to be passed to the baby. Usually you can get fecals done without taking the hedgehog in, so it'd be a good first step to making sure everything's okay without stressing her/baby out with a vet visit right away.