Based purely on conjecture, along with a few recent statements, my guess is that Friendly C will be the US vs. Columbia on October 12.
From SI.com: "UP AHEAD: While the U.S. usually takes off most of the six months following a World Cup, this year's schedule is unusually busy. An Oct. 9 exhibition against Poland at Chicago is likely, followed a match against Colombia three days later, probably at a stadium included in the 2018-22 U.S. World Cup bid, as the New Meadowlands is."
I am guessing that such a game would be a pretty big success at Qwest. First of all, it is unlikely that any big-name club is going to be in town that is going to sell many tickets. Those teams have left or are leaving before any such game could get organized. Second, the FO has not quite embraced the #trophiesnotfriendlies campaign in any meaningful way. Third, with his form, I could easily see Fredy playing, if not starting, for Columbia. Not only would Sounders fans be treated to the USMNT (which would admittedly probably be playing younger fringe guys) but they would see their budding star in an international game. This makes a lot of sense, and is one friendly I would support.
One kink in my idea is if the FO is sitting on the USOC final. Lets say they are set at getting US vs. Columbia into Qwest, but also want to embrace the #trophiesnotfriendlies campaign. If we get into the final, they might award that game as Friendly C. Since we are not there yet, they don't want to make an announcement one way or the other on either game. This, in my mind, is quite possible. I'm just hoping I get to see both games.