I think the IRS scans all envelopes that they receive.  That means if there was a postmark on the envelope, the IRS should have a scan of it buried in their computer somewhere.

He can try calling the IRS to find someone who is willing to look up that scan, but my guess is that it won't be easy to find somebody that is willing and able to do it.  It sounds like about $900 of work to get rid of $850 of penalties and interest.