A pharmacist may be able to provide that info for you. I wouldn't go to the pharmacist that provided the two drugs for your mom though as, they may not be forthcoming while trying to cover their own bottoms. It is, as far as I understand, a pharmacist's job to make sure that all medications prescribed can be mixed/used together and to make you aware of any potential problems or side effects. I could be wrong about that but, it seems that alot of companies like CVS, Target and Eckerd have done commercials about the fact that they will make sure your prescriptions are all compatible.
Have you considered contacting a malpractice attorney? Or, your local medical board?
Take care and I am sorry for your loss.