The dose of lactulose is typically 15 to 30 ml at a time and that would simply too much to take in via a capsule. Pharmacies DO sell empty capsules but with such a volume, you would have to ingest over 10 capsules per dose.

Lactulose is known to be tough to tolerate.  To improve flavor, mix lactulose with a full glass of water, milk, fruit juice, or carbonated citrus beverage.
One alternative could be to try and have her take a drug known as Kristalose. It is a powdered form of lactulose that can be mixed and tends to be better tolerated.