Did you have that conversation with her in the morning at the breakfast table, or in her room? The breakfast table scene is graphically broken but works (at least for me) but I've read that this bug occurs in her room.
And they claim to have fixed it with 0.18.2.
I've actually got it to run without problems.....I got it by completing Jenny's route first, before doing anything with Roxy.
I actually had the scene in the afternoon, in Jenny's room.....Although first part it said that I needed to get the money, even though I had it...All I did was when it told me that and ended, I re-clicked on Jenny and ran through the Roxy cheerleading message again and it then gave me the option to actually pay her. Thus letting me go back, tell Roxy(at noon) and upon coming back home the same day, it started that event.
Also note..that I waited to impregnate Jenny till I passed Roxy's cheerleading part....also I'm running on the cheat game, as I didn't want to bother with that side story and mini game shit. Plus I'm using Laim's mod, absolutely love the fan made stuff and the walkthrough part of the mod is a must.
I must also add, I only did the Jenny and Mom/Debbie routes to completion, before I moved to other characters....maybe did a very small part for Aunt Diane at the same time.
It seems the game runs way better doing one character at a time, and as Roxy intersects with Jenny, do Jenny first before tackling on Roxy.
Maybe that will help...…………….Peace!!