Not sure if my friend is listed as co-owner or beneficiary of his mothers SDB. He had a key to the box but he was only able to remove the will. His mother was a
Widow. Is his normal practice so that the state an feds can get their hands on the
Taxes?

No, it has nothing to do with a nefarious plot on the part of government to "get their hands on the Taxes" (sic).

It's the agreement the bank makes with the owner of the SDB (your friend's mother, in this case) when the box is first rented.

If he was allowed to open the box, the bank may have relaxed the rules of the agreement, or it may have specified that he could open it for the limited purpose of retrieving the will. Hard to say.

If the person was also a owner (co/owner) then they could get into it,

I am surprised he was even allowed to get the will, without a order from the court.

But, no, it is because all items has to go though probate court. So that it is done properly