It is ultimately the physician's and pharmacist's responsibility. As a pharmacist, I can tell you many pharmacists/pharmacies are overworked and understaffed. The computer software is what is usually catching the drug interaction. Moreover, there are few studies of meds in geriatric populations, especially those over 75. In addition, the polypharmacy that occurs in many Americans is quite scary. It is frankly quite difficult to comprehend the impact of multiple rx meds, OTC meds, and who knows what supplements. Last, there are many types/varieties of drug interactions (e.g. drug-drug, drug-disease, drug-genome) to name a few. 
Feel free to shoot me a PM if you have any questions about your Mother's condition. Sorry, to hear that she isn't doing well.