The 24-year-old joined Werder in 2002 but has not had the chance to establish himself at the Weser Stadion after suffering two cruciate ligament injuries since his arrival.
A return to his former club would seem to be the best option for both Friedrich and promotion-chasing Mainz.
"After his bad injuries going to Mainz will give him the chance to get in some real match practice - something we can't offer him here at the moment," explained Werder sport director Klaus Allofs.
Friedrich, who is contracted to Bremen until 2006, expressed his delight at the opportunity, saying: "I will do everything possible to develop as a player over the coming months.
"If I can help Mainz get promotion in the process that would be great."
