Yes, you must have her libido high enough (it might be lower than 10, but it needs to be high) to see those scenes. It's been a little bit since I did the Truth or Dare with Mandy and Lauren, but you should definitely finish that. Generally, there are story events throughout the game that will help increase Mandy's libido. (The cosplay picture shoot is one example. In addition, if you
save both girls from Agent Diaz, both Lauren and Mandy reward you and increase their libidos as a result.)
After finishing Truth or Dare, you'll have the option
of stopping by at night to play more games. During one of those later visits, you for sure can increase Mandy's libido (+3 I think?), so that will help you as well. Note that after you see someone cum (through masturbation or sex), their libido drops quite a bit, so you'll need to build it back up again. Fortunately, Mandy is naturally a
very sexual creature, so there are many ways to arouse her libido so she'll seek some release.