Yeah she literally says "I don't wanna do Frankie's bits, they're too high for me." Right before they realise they can't sing Missing You because Frankie's pre-recorded vocals are baked into the performance track. Whoops. I'm assuming they gave her those lines for Faster because it was (technically) in her range and required a bit of power? If Rochelle did it she'd be singing 90% of the song, and Mollie wouldn't have the power for the belting bits.