When they do a stress test, they monitor the patient continually, blood pressure included, so the likelihood of anything going wrong is very slim. I would get the test done soon, to get it out the way, then your wife doesn't have to keep thinking about it and worry more.
You should be able to accompany her, and sit next to the bed during the procedure. I did this for my wife a few months back and cracking silly jokes during the procedure helps to lighten them up. She sounds very anxious about the test and this is probably giving a lot of input to the high blood pressure problems.