I know, that's why i said someone. He will not confront her as of now. But he did not say anything about not being on the PTA trip, but rather, that "shouldn't be here"( maybe not a place, but rather involved in this whole ordeal).
Terauchi is probably involved, but i don't think it's him that Sousuke is looking for( maybe his function is to actually paying attention to Sousuke at work, and somehow, one reason or another, he ended up pointing to Momonga. In a classic Hero's journey analisys, he's the call to adventure in the Story).
My take is the last guy, Kamizaki. The one that works on a bank( the second glasses guy, not Isobe, the one who got cucked by Shiratori).
In chapter 01, there's a shadowy figure talking about it being the first time he saw Rika's husband, that he looked as dumb he heard and even brought a treat to her lover's affair partner. If it was Shingo, he would be still talking to someone else. We can assume now that there's a 3rd party.
And more importantly, we never got full acknowledgement that Rika's income with Momonga's( and maybe Shiratori's site) got her enough money to pay for Matsuo's business startup. She probably has other sources of income.
He probably helps manage her account, and considering we know her secret account exists, and she and Shingo are trading information through financial apps, his name may have appeared to Sousuke on Shingo's phone. A common contact to both he and Rika.
And the bank guy could fit well in trying to "tie the knot" on these issues, as well as point out a next direction to follow and try to find more of her schemes, if "defeated" like Shingo or Shiratori.