My mom is taking a pain killer called hydrocodone and she needs to know if it is safe that she takes excedrin?

Tylenol (acetaminophen) toxicity. She needs to be careful if getting excess tylenol. I would avoid any over the counter pain reliever containing tylenol (acetaminophen).
Do not mix. It is not a good idea to mix otc medication with your presciptive drugs without clearance from your physician. For instance, most if not all typical hydrocodone compounds have Acetaminophen and so does excedrine. Potential exists to harm the liver.