I seem to have half resolved this issue. Following a hint in another thread, when "Use SSL for progressive downloading" is checked in the FileMaker Server admin console (under 'Database Server > Security'), the customer was able to view PDFs in interactive container fields. Turning this option off again, reproduced the problem. Therefore, it appears to be the connection to the server that was causing the issue rather than a local conflict on the laptop. I removed the favourite host that the customer was previously using to open their file, and instructed them to click on the local host instead, which enabled them to view PDFs within interactive container fields WITHOUT "Use SSL for progressive downloading" checked.
I still need to follow up with some issues relating to remote connectivity (still not allowing the customer to view PDFs in interactive container fields) and why this doesn't seem to be a problem for many other customers.