A parathyroid tumor (adenoma) won't go away on its own. Once you have it, the only way to get rid of it is to remove that parathyroid.
Although PPI use is connected to hyperparathyroidism, surgery would still be necessary since the tumor (if there is one) on the second parathyroid will not go away.
What are the actual calcium and PTH levels? Excess calcium can lead to kidney stones, irregular heart beats and brain issues.
High PTH can also be caused by condition that cause low phosphorus, such as chronic kidney disease.
Make sure the surgeon doing the surgery is very experienced and looks at the other parathyroid glands as well. Most of us have 4 of them that reside behind the butterfly shaped thyroid gland. Since you mother has had one removed, she should still have 3 left. If another is removed, the surgeon should make the 2 remaining parathyroids do not also have tumors.
I don't have an answer for you, if she is supposed to be on the medication I would talk with the doctor first before stopping.
Can parathyroid hyperplasia or a tumor be reversed if it is being caused by the Nexium (or other proton pump inhibitors)? If it is at the point where they want to remove it, I would assume that it is enlarged and producing too much PTH, and stopping the PPI at this point may be too late to reverse the increase in PTH.
I'm not at all an expert on parathyroids and suggest you discuss this with your doctor and especially the surgeon who hopefully is very knowledgeable about this topic.
I did find some information on a parathyroid website: https://www.parathyroid.com/causes.htm which states that
"What Causes Hyperparathyroidism Cause #2:
TWO of the parathyroid glands grew into a tumor. This is 26% of all parathyroid patients -- they grow TWO tumors for no known reason. No Known Cause.
This is just like the section above, except that there are two parathyroid tumors and two normal glands. One of the parathyroid tumors (adenomas) will be bigger than the other, and usually the bigger (older) one is the one that shows up on a scan. If your surgeon removes only one of the tumors leaving two normal glands and the other tumor behind you will be a bit better, but not cured. You will need a second operation (see our page on re-operations following failed parathyroid surgery). Now you are seeing that parathyroid surgery can be a bit complicated and why experience is the name of the game. This is why we look at all four parathyroid glands when we perform this operation. If your surgeon only removes the ONE parathyroid tumor that they see on a scan, then you will have a 15-20% chance of needing another operation to remove the second tumor some months or years later."
It doesn't rule out that the cause in the first place was PPI, but suggests that surgery might be necessary to get the hyperparathyroidism under control. And, it sounds like a second parathyroid tumor is very common (1 out of 4), often with one gland larger than the other and that second one often missed during the surgery.
Good luck, and I'm sorry your mother has to deal with the possibility of another surgery.