If it's the same sort of 'assessment day' that I attended for a possible car sales job your nephew can forget all about traditional interviews.
The event I attended had nearly 100 people there for the first stage. We were divided into groups of about 8 or 10 people, who were then asked to work together on a particular task. I seem to remember that it was something to do with being stranded on a desert island, with various items at our disposal and being asked to prioritise our needs and actions. Groups of assessors moved between the groups, noting who was contributing to the discussions and how they were doing it. (e.g. who was taking the lead, who was being too reticent, who was trying to dominate the discussion, etc).
The assessors then called 'time' on the initial discussions and asked the groups for their conclusions. (Again, I assume that they were looking to see who wanted to be the spokespeople for each group, and whether they were representing everyone's views or just their own).
At the end of that stage, about 60% of the participants were thanked for their attendance but asked to leave. Unfortunately I can't tell you what happened after that, as I was one of the 60% !
Chris