Dansby Swanson hit his 17th homer for the Cubs (65-47), who fell three games behind NL Central-leading Milwaukee.
With two outs and runners on the corners in the seventh, Friedl drove in Jake Fraley with a looper into center field against Caleb Thielbar. Ryan Brasier (0-1) got the loss.
The Cubs threatened in the bottom half, advancing Willi Castro to third with two down. Swanson then hit a chopper to third and was ruled safe.
The Reds challenged, and the call was overturned when a replay review showed Swanson missed the bag initially as he crossed first base.