I am presuming a few things here, so let me assess based on this:
if he wants to double his FCI to 15k miles, then he's running the first OCI to 7.5k miles.
I'll remind you all that oil gets better with age, too. Or, more specifically, the oil add pack ages and creates a desireable barrier on the parts. Old oil is good oil. So if you're going to leave the filter for a second OCI, why drain the oil.
OK - I can hear the jaws dropping and hearts fibrillating ...
Let me be clear here. I'm talking about oil that is still in a usefull cycle, not run past any hope of salvation. For the prescious few of you that have actually read the SAE article I reference (2007-01-4133) time and time again, you understand. For those of you that don't read it, then you'll forever be lost in the minutia of sales rhetoric any mythology.
I am NOT advising someone to over-run any lube. What I'm saying is that if the lube was still good when it was dumped (and it probabaly was) then why worry about what's leftover in the filter?
There a few OEMs that clearly advise it's OK to 2x for FCI. They never state to remove it and drain and reinstall. They just advise to let it stay and run for the second OCI.
But hey, why left facts and common sense get in the way of a good myth anyway?