Naive question, but I'd like your input. In the movie, after Juan brings Eva home, he waits outside the bedroom and lets Evita "evict" the girl. Why couldn't he do this himself? Is it different in the stage production (which I'd love to see)?
In reality, Juan wasn't present. Eva had her own apartment, which is where she and Peron retired to after meeting at the earthquake benefit. Later, Eva discovered he had a "daughter," who turned out to be his teenage mistress. Juan never sired any children, but he used to introduce his mistress as his "daughter" and even had a nickname for her -- piranha!
Anyway, a few days later Eva showed up at his place unannounced and sent the girl back to her hometown of Mendoza. It is not known for sure what transpired or what was said between them, but it couldn't have been a friendly encounter. When Peron returned, Eva had situated herself and he more or less accepted it.
On stage, the action needs to move along, so Eva and Juan go back to his place instead, where the unsuspecting mistress awaits them. And that's why Peron is in the scene in this version.