Sounds like he was hedging on interest rates.  For example, he chose a variable-rate mortgage and then bought a futures contract on Treasury notes that would pay off when the Fed did its thing.  So now his mortgage payment has gone up, but he has $100K to make himself feel better.  Can he mark to market?  Should he?  There's a lengthy article here that might help:

https://www.thetaxadviser.com/issues/2010/feb/sec475mark-to-marketelection.html

Depends on whether he is classified as a trader, or an investor.  It might not make any difference, if he closed out the contract (which he probably should have done by now, since the Fed has probably reached its peak).  Does he do any other commodity trading?