I was just in the US for Thanksgiving, by myself, and it's easy as pie.
The line was ridiculously long for non UK and EU passport holders (all of us on BRPs) the day I arrived at Heathrow. Stood around for about an hour, got to the front, handed over my passport, BRP card and landing card. He asked me a question, maybe two. I had a terrible migraine and looked really dodgy I'm sure, and I walked through to my baggage and went to catch the tube to my train. It's a bit anticlimactic when you're worried. Haha.
She just has to make sure she has everything filled out (landing card) and her passport and BRP and she'll be fine.
When the landing card asks how long you plan to say you can say whatever. I put "indefinitely (I live here)". Haha