You need to read the packaging info of the product. Call the doctor who prescribed it and ask. It may be that they'll prescribe something for the stomach upset or eliminate Aricept immediately.
You can stop giving it to them and notify the doctor that you did and why but maybe he'll try something else.
My Grandma experienced severe stomach pain and vomiting with Aricept. After speaking to her Dr. we discontinued the medication.
We also tried Aricept with Mom and her side effects were increased agression, insomnia and her confussion became much worse. After speaking with her Dr. the Aricept was stopped. I wish they would have responded better to this medication as I've heard others have but they did not.
Good luck to you and your patient.
When is it being given to them? In the morning? If so, you could trying giving it in the evening rather than in the morning so that it doesn't cause so much tummy upset.