Sorry to hear about your friend's experience.

Government interrogators -- even of the low-brow variety at immigration -- make a habit of lying to either catch "you" in a lie or to get "you" to say things that they don't know. Never believe ANY claims being made during an interrogation -- especially not about "he/she said".

The UK is notorious about harassing young females far more than young males visiting romantic interests. And directly or indirectly accusing women, especially from some countries, of being prostitutes is not unusual.

Is she a 40-something US-born American national?