He accomplished his post doctorate during the years 1971-1973 at the Institut of Mathematics, Heidelberg University, with Peter Roquette as his mentor, and habilitated there in 1972.

During these years in Heidelberg, he initiated an intense and long term cooperation with German mathematicians, especially with Peter Roquette, Wulf-Dieter Geyer, Gerhard Frey, and Juergen Ritter.

One of his great achievements is the publication of the book "Field Arithmetic" in the series Ergebnisse der Mathematik und ihrer Grenzgebiete of Springer, which earned him the Landau Prize.

This was during this visit that Fried suggested to write a joint book on the topics they had worked about so far, and proposed the name "Field arithmetic".

Some years after the release of the first edition, it was realized that the term "field arithmetic" had already been coined by the mathematician Paulo Ribenboim somewhat earlier, who had published a book in French named "L'Arithmétique des Corps".