A friend received a claim she owed money for subscription that she didn't recognize. What is best way to proceed?

Ask, in writing, that whoever sent her the claim to supply documentation of her order (subscription).

If she needs to call someone about this, keep notes of who was called, when, substance of the conversation, etc. Confirm any conversation by a written letter (or e-mail).

She should also monitor her credit reports; in case they say she hasn't paid what she owed, she will be able to dispute it with the documentation request (and response, if any), and with the records she has kept as suggested above.

Make sure you dispute all claims in writing and ask for documentation. Keep copies. You have 30 days to dispute claims for payments.

Many magazine subscriptions require you to cancel them formally and are sent automatically when you don't, along with a bill. Typically they are part of a free trial or offer that you can cancel anytime and don't require a credit card.

The required cancel notice comes with the free magazine.

https://www.consumer.ftc.gov/articles/0101-free-trial-offers and its not limited to magazines either.

Nation's Top Scam? Free Trial Offer - CBS News (http://www.cbsnews.com/news/nations-top-scam-free-trial-offer/)

'Free trial' or Internet scam? - Dec. 17, 2009 (http://money.cnn.com/2009/12/17/pf/saving/no_free_trial/index.htm)