I had asked for some further information and have not heard back.
This issue is far more complex than a simple yes or no question of whether it is really dangerous. There clearly are some risks, but it also is not universally dangerous.
First, it is important to note that the greatest risk of hormone replacement treatment (HRT) is combined HRT, which includes estrogen and progesterone. However, in women that have had a hysterectomy, such as yourself, we are able to use estrogen alone, and the risk of estrogen alone is much less. If you read articles about the risks of HRT, it is important to separate the risk of combined HRT from estrogen alone HRT.
The primary concern with estrogen alone is a small increased risk of breast cancer and an increased risk of stroke. Part of the concern, though, is that your risk for both breast cancer and stroke is much higher now, at age 70, than it was when you were started on estradiol, at age 50.
Combined HRT has a significant increased risk for heart disease, but estrogen alone does not appear to have that risk, and may actually lower heart disease risk slightly.
The general recommendation is that estrogen replacement should be avoided when it can. But if a woman has significant menopausal symptoms, including hot flashes, and these symptoms cannot be controlled by other methods, it is OK to take estrogen replacement. When estrogen replacement is done, it should be used at the lowest effective dose and for the shortest period of time that is effective, but if the symptoms recur when there is an attempt to stop the estrogen, it is a sign that it is still needed.
I asked for the additional information because this information pertains to other methods for controlling menopausal symptoms. Certain antidepressants have been shown to help each menopausal symptoms. The Wellbutrin, though, is not one of the antidepressants used for this purpose. The antidepressants that are more commonly used include venlafaxine, paroxetine, and fluoxetine. The Wellbutrin can be used for conditions other than depression, but if used for depression and you have not previously been tried on one of these antidepressants, it may be worth trying a different antidepressant. There also are certain anticonvulsants that can help to control hot flashes, such as gabapentin or pregabalin. These medicines are also sometimes used for restless legs syndrome (although there are many other medicines that can be used for restless legs syndrome).
In summary, then, if you are having persistent hot flashes and they cannot be controlled by other methods, then resuming the estrogen is reasonable. There is a small risk, although greater than when you started the estrogen replacement, but it should not preclude the use of estrogen if she is having significant symptoms that cannot be controlled.
If I can provide any additional information, please let me know.