I have seen many cases a lot worse. Around the 300/200 range or higher and the people were still walking around. Not sure how. The biggest worries with high blood pressure are with possible kidney damage, arterial damage leading to arterial plaque, and the risk of blowing out an aneurysm.
The magnesium is a natural calcium channel blocker. So I would add ashwagandha, which is a natural ACE inhibitor and beta blocker. That will cover most causes of high blood pressure.
Another great herb is coleus forskohlii, which works like digitalis, even though it is not a cardiac glycoside. Forskohlii slows the heart, strengthens the heart contractions, and lowers the blood pressure. Take it with nettle leaf or watercress to prolong the effects.
Sterols also lower blood pressure by relaxing the blood vessels. Jiaogulan is a great source.
And there is the amino acid taurine, which also relaxes blood vessels. This must be taken on an empty stomach at least 30 minutes before meals.
Those are my choices in order of preference with the magnesium.
If she still has the hyperthyroidism then this could be contributing. The herbs bugleweed and Melissa (lemon balm) are both excellent for suppressing the thyroid in cases of hyperthyroidism.