Why “High” Blood Pressure Is Sometimes Okay

I am not sure where to direct this question, but my mom-in-law is 93 and the doctor has told her to monitor her blood pressure at home for 2 weeks. The family bought a BP monitor. She finds the pressure too sensitive and will not allow anyone to take her BP. It hurts her arm, she says. She is very tiny and barely weighs 100 pounds on a good day.. At the doctor's office her BP ws 170/100. Mom also has severe arthirits and has no cartlage in her knee. Very paintful and her leg is swollen and bruised. If anyone has experience with taking BP for the elderly, I am interested in what works best for them. Thank you...