Frei returns as Basel bag Swiss Cup
Frei, Switzerland's record scorer with 40 goals in 73 appearances, came on midway through the second half for his first match since breaking his arm during a league game in February.
Valentin Stocker and Xherdan Shaqiri scored twice in three minutes during the first half to put FC Basel in control and Cameroonian striker Jacques Zoua ended the game as a contest by scoring less than a minute after the restart.
Australia midfielder Scott Chipperfield, Stocker and Switzerland midfielder Benjamin Huggel added further goals to give Basel their 10th Cup win and keep them in with a chance of the double.
Basel are three points behind leaders Young Boys Berne in the Swiss Super League with two matches to play.