High Disk I/O from sql server or is High disk I/O slowing sql server?
Have a look at the following perfmon counters:


SQL Server, Buffer Manager Object:
Page lookups/sec
Page reads/sec
Readahead pages/sec  
SQL Server, Access Methods Object
Full Scans/sec
Range Scans/sec
Skipped Ghosted Records/sec  
SQL Server, Wait Statistics Object
Page IO latch waits  


SQL Server driving a high number of IO requests would be corroborated with a high number scans, increase in page lookups and page reads and high page IO latch waits. Is worth trying a look at sys.dm_exec_query_stats for entries with high physical reads counts. They could quickly pinpoint the culprit.

In general approaching the problem as a performance troubleshooting problem, following a methodlogy like Waits and Queues is the right approach. You DBA seems to be doing the right thing so you should listen to him.