Given what he does in the locker room, it seems entirely plausible that everything she said was true.To be fair, for last update specifically we only saw what happen in the gym itself through Amelia's recollection while they were reclaiming. It's entirely possible she was playing it up for effect.
For people who might still not see the issue here, imagine that the MC hadn't stumbled on them in the locker rooms. What would've happened? Brandon seemed intent on having his way with her, even with her saying she wanted him to stop, so she was going to end up raped, or she was going to break free and escape, and likely have been traumatized by the situation. Also, she might've also been found by someone else and suffer further humiliation - oh look, there's the woman that was fucking Brandon in the lockers. This is pretty bad, man. I don't know that you could redeem the situation with a few extra lines.