The risk would be greater if she were doing an exercise stress test. She will will experience a period of some discomfort, but less than a usual angina attack, at least that is how I can explain it. The risk is less than the reward, I'd procede with confidence as the risk is very small. Baldie453 gave very good advice, in my opinion.
I have had a chemical treadmill without any issues. My Mother, who will be 86 this year, has had this type of stress test for many, many years, without any issues. All stress tests have some risk associated with them, but there are always personnel on hand to handle any issues, should they arise. Just tell your Mom to relax and all should be fine. Good luck!