Okay, not exactly sure what happened but we managed to force the task to go through by inviting Andre to hang out a few times in a row, week after week, with just skipping in between.
We did 2 saves in your playthrough: 6-1 is On Friday night right before the scene finally started working perfectly, and 6-2 with the required "Hang out" scene already started (saved on the first dialogue line). You can download the saves here and drop them in the same folder we mentioned before.
To try and replicate the "fix" (Might not work since we don't know what's causing the issue):
1. Sleep every day to go straight to Friday night and when asked how to spend the weekend, choose "Hang out with Andre".
2. Play/skip the scene in the pool.
3. Keep sleeping every day to once again go to Friday night to again "Hang out with Andre".
4. Play/skip the scene in the pool.
5. Sleep through the week for the last time to get straight to Friday night and this time around "Hang out with Andre" will actually trigger the right scene.
Not sure what's wrong with your save, but we hope you can enjoy the best girl route in the game while we're taking a closer look at your save file to figure out why it's bugging out!