Tips on undoing bad behaviors in 5yr old

My boyfriends daughter is turning into a nightmare. She’s extremely disrespectful to adults and is very demanding. She also cannot be in her room by herself for longer than 30 seconds with out attention. It’s not my place to tell him how to raise his daughter so when she is rude directly towards me I check her in a calm but firm tone with responses like “you don’t speak to adults that way” or “you need to ask politely for that, you can’t just take things” or a simple “no” when she’s extra bad. She also has an instant and extreme “fake cry” when something doesn’t go her way. He doesn’t see her often so he doesn’t really correct her, I would appreciate some other input on how I can help the situation. It’s very confusing. He wants me to have a “role” with her but it’s hard when I don’t want to over step boundaries. I feel like there should be certain rules when she comes to my house that aren’t respected when she goes home with her mother. Help.