Disk IO? How they confirm this. Do they have any IO related error in event log ? .

select * from sys.dm_io_virtual_file_stats (Null,null)

Run the query to see any IO stalls (IO_stalls_read_ms and IO_stalls_write_ms) are

there (considerable IO stall for considerable duration)

You may run Performance Monitor.exe to find disk IO metrics.See for "Disk Queue Length".