The testimony is expected at a preliminary hearing for counselor Stacey Strauss of Philadelphia. She is charged with involuntary manslaughter in the July 24 death of Bryan Nevins.
Police say Strauss left the severely autistic Nevins in the van that 97-degree day when the group returned to Woods Services in Langhorne. They say she also made conflicting statements after his body was found.
Defense lawyers argue that many co-workers share blame for the New York man's death.
The nonprofit Woods Services serves about 1,400 clients from several states at facilities in suburban Philadelphia.