Does Detach/Attach or Offline/Online Clear the Buffer Cache for a Particular Database?
I initially thought you were on to something here. Working assumption was along the lines that perhaps the buffer pool wasn't immediately flushed as it requires "some work" to do so and why bother until the memory was required. But...

Your test is flawed. 

What you're seeing in the buffer pool is the pages read as a result of re-attaching the database, not the remains of the previous instance of the database. 


  And we can see that the buffer pool was not totally blown away by the
  detach/attach. Seems like my buddy was wrong. Does anyone disagree or
  have a better argument?


Yes. You're interpreting physical reads 0 as meaning there were not any physical reads


  Table 'DatabaseLog'. Scan count 1, logical reads 782, physical reads
  0, read-ahead reads 768, lob logical reads 94, lob physical reads 4,
  lob read-ahead reads 24.


As described on Craig Freedman's blog the sequential read ahead mechanism tries to ensure that pages are in memory before they're requested by the query processor, which is why you see zero or a lower than expected physical read count reported.


  When SQL Server performs a sequential scan of a large table, the
  storage engine initiates the read ahead mechanism to ensure that pages
  are in memory and ready to scan before they are needed by the query
  processor.  The read ahead mechanism tries to stay 500 pages ahead of
  the scan.


None of the pages required to satisfy your query were in memory until read-ahead put them there.

As to why online/offline results in a different buffer pool profile warrants a little more idle investigation. @MarkSRasmussen might be able to help us out with that next time he visits.