Robert changes back and forth from Heidi whenever he gets aroused; though climaxing also causes the change. Heidi had sex with her best friend, changing back to Robert after the climax; forcing Robert to make himself to climax again, to change back and hide the truth.
Seeing as Robert's mother turned into him when given the drug, Heidi is likely similar to what she looked like young.
Zelda notably is good at multitasking; being able to focus her phone call with Robert while having mild sex at the same time.