The suspected instigator of the assault was described as a woman in her 40s with two young children, Chitwood said. She is believed to be the stabber, he said.
Around 5 p.m. at the McDonald's, in the 100 block of South 69th Street, customers complained to the manager that two women had entered a second-floor restroom designed for one person.
The manager entered the restroom, caught the women engaged in "sexual relations," and ordered them to leave, Chitwood said.
As the women tried to do so, they were verbally accosted on the second floor by the woman with two children and others.
The pair were trying to leave the McDonald's when one was attacked. The second woman was stabbed when she tried to help her friend.
By the time police arrived, the attackers had scattered and the victims - Delaware County residents - also were trying to leave. Police persuaded the woman with the stab wound to go to the hospital.
Chitwood said investigators were reviewing surveillance video from the McDonald's and a nearby Rite-Aid store.
Contact Robert Moran at 215-854-5983 or firstname.lastname@example.org, or follow @RobertMoran215 on Twitter.