Credit Card Company's Fraud Department Called Or Not?

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When you get a call saying it's from your credit card company's fraud department, how do you know if they aren't an impostor? My friend told me she got a call saying it's from her credit card company's fraud department and they ask her in an automatic voice whether she authorized charge of money for a certain transaction and gave her options to choose from like press 1 if you authorized this transaction and press 2 if you did not authorized this transaction or you aren't sure. The transaction was with a company she used her credit card with to sign up for a free offer. She shouldn't be charged any money and there shouldn't be a transaction with the company because the offer is free. She pressed 2 saying she did not authorize the transaction and they directed her to another line to speak with a representative but the line was busy so she was put on hold. But after being put on hold for a short time, she hung up because she thought they were an impostor and thief trying to get her personal information. The transaction hasn't showed up on her credit card so she doesn't know whether the company really charged her money on her credit card when they shouldn't have and whether her credit card company's fraud department really called. What would you do if you were in her shoes? How should she handle this situation? Please give some advice. Thanks.