I would take her to a GP just to be sure there is nothing else underlying there, particularly due to her age.

Are you certain its the dairy protein causing it? If so, are you able to avoid it long term to stop this issue happening again (sorry no idea about that particular allergy) or is the constipation likely to become a long term problem because you can't completely avoid the allergy due to her age?

A GP can also go over the pros and cons of long term use and may be able to advise you to something different if needed. There are so many products out there for this type of problem but they all have their benefits and risks that someone should go over with you.

Im not sure if this helps at all but as far as I was aware in Australia the top eight allergens have to be listed on any food products that contain them - Im not sure if this extends to medications.

I hope she feels better soon.