Even at the prep mansion, she still doesn't get it on with the mc unless he makes a donation to the HOTs (i.e. Quinn's) cause.
Even Sarah, who's all about the cash, gives the MC a free blowjob in the hot tub, helps the mc when he's fucking Mel in the shower, and if the MC has shown interest in her (i.e. commented on her rather than Mel or Heather in the hot tub, or chose Sarah as the best ass on the movie night) will fuck him for free after (actually during) gender studies.
And then there's Lily, who's all business. She'll still give the mc a free blow job if she thinks he's sincere (i.e. a Chick kinda guy). I do't think grinding the MC's cock in the sauna counts though, she was commanded to do that by her mother.
But Riona? All you get out of her is a kiss at the DIK's mansion during the Cathy Cluck fiasco, and maybe you can count 5 seconds of a foot job (but the MC still had to pay for Sarah to get there).
Come on Riona, I love you and all, but if you're ever gonna cum during sex, you're gonna have to take your mind off the moolah!