Truly sorry to hear about the accident.
The only stairs at O'Hare I can think of that are publically available are the ones that lead down to the T1 T2 shuttle, either at F or C concourses. But those aren't near bathrooms, if I remember right.
There's also a (seemingly) private staircase on the B concourse somewhat near the bathrooms, but not immediately outside.
Anyone have any ideas? Are you sure this incident was in the United terminal?