Maybe he was talking about the Twelve Passion Gospels of Great and Holy Thursday?
What is this?
The pericope of readings on Holy Thursday evening (which is Matins of Holy Friday) - 12 Gospel readings all focused on the Passion and Crucifixion. All the readings come from the traditional 4 gospels (i.e. not from the Diatessaron), but there could have been a confusion on the student's part...
The student may have just been referring to gospel harmonies in general rather than the specific Diatesseron.
If that's the case, then the Orthodox do have Gospel readings in Holy Week taken from all four Gospels which could be considered Gospel harmonies.