He says, I am a dentist, and all medical studies have proven that the best material for crowning the back teeth in terms of strength.

And for protecting the tooth for a long time is gold. So is it permissible to use gold crowns for the back teeth for a medical, not cosmetic, purpose? There is no problem with that, insha'Allah, and the Prophet ﷺ, during his time, a man was injured.

His nose was broken, so he had a nose made of silver, but after a while it started to smell, so the Prophet ﷺ allowed him to make it from gold. So if it is out of necessity, there is no problem.

So he may use gold for the teeth if it is out of necessity, there is no problem with that, and Allah knows best.