How widespread was that taught by Tetzel in regard to indulgences for the dead?
The old Catholic Encyclopedia says:
As much cannot be said about his teaching regarding indulgences for the dead. The couplet attributed to him —
As soon as the gold in the casket rings
The rescued soul to heaven springs,
like that attributed to Luther,
Who loves not wine and wife and song
Remains a fool his life long;
though verbally spurious, can in both instances be in substance unfailingly traced to the writings of their respective authors. By Tetzel they are substantially acknowledged in his Frankfort theses. Here he accepted the mere school opinion of a few obscure writers, which overstepped the contents of papal indulgence Bulls. This opinion found no recognition but actual condemnation at the hands of authoritative writers, and was rejected in explicit terms by Cardinal Cajetan as late as 1517-19.
The New Catholic Encyclopedia says:
Tetzel was orthodox in regard to indulgences for the living. In regard to those for the dead, however, he followed the teaching contained in the Mainz Instruction issued to preachers of indulgences. That is, he taught the then widespread, erroneous theological opinion that indulgences for the dead were gained independently of dispositions of contrition in the person seeking the indulgence, who also had the right to apply them absolutely to a specific soul in purgatory. Cajetan condemned this teaching at Rome.
Now that's historical clarity!