It was the devil's threesome flag that was set but I think that boat would probably have sailed if you do not help Trent with Eri - personally, I want Eri (teacher) myself so didn't offer to help Trent even tho he is a pretty good guy.Earlier Trent expressed interest in sharing Eri with MC if you chose the right dialog choice. So that might be an option later with Eri and/or Bridgette Lopez.
Also Trent thinks his mom is a slut and would never bang her as he doesn't want to cheat on Eri even if it is all in his head.