If you fix the post nail (that is, the steel nail you mentioned) on both teeth, you need to do root canal treatment (I wonder if your mother has done root canal treatment on both teeth), then put down the nail and finally make the porcelain crown. To put it bluntly, it costs a lot of money. But the effect is quite good, because the nail is firmly fixed on the root.

I suggest that if the teeth on both sides can still make porcelain crowns, make three more. In this case, you don't have to damage the root, the root is still your own. If both teeth have been treated with root canal therapy and have no dental nerve, it is better to make porcelain teeth with post crown (that is, the kind with nails), because their teeth have no nerve, they will become brittle in the future and the next nail will be stronger.

So in your mother's case. I believe that after reading my suggestion, you have made a choice.