A close family friend recently had plans to visit my sister who lives in the US. She's a Canadian citizen and moved to Canada from Iran when she was four years

A close family friend recently had plans to visit my sister who lives in the US. She's a Canadian citizen and moved to Canada from Iran when she was four years old. Anyway, she's been to the US numerous times, but was denied entry two weeks ago, with no reason given, despite the ban not being in force.