Yes, they should worry a lot -- there is an increasingly common set of steps that copyright owners typically use to identify illegal downloaders these days. It may be different or more difficult due to the distributed nature of torrent downloads, but here's the basic way it happens.
First, sophisticated technology is used to identify the IP address of a computer or other device used to download copyrighted content. Next, copyright owners request that internet service provider (such as your cable company or whoever provides internet access to you) identify the internet service subscriber associated with the IP address for the particular day and time of the download. Sometimes the internet service provider provides the information immediately, and sometimes they give their subscribers notice of the demand so that the subscriber can seek legal relief from the request by going to court. Once the copyright owner identifies the subscriber, the normal practice is to send a letter threatening to sue unless a settlement is accepted of at least several thousand dollars. This isn't particularly fair since the subscriber isn't necessarily the person that used the computer, but it's the reality of how things work. (In fact, the subscriber is often a parent of a teenager that downloaded the content.) Nevertheless, it becomes an incredibly difficult and expensive situation to deal with once someone is identified and targeted by a copyright owner. There are a number of law firms and other entities today that specialize in exactly this sort of thing.
Your friends are correct that a lot of people today download copyrighted content without permission. They are very misguided, however, in thinking that they definitely won't get caught. It might take a while, and it might not ever happen, but the chance of them getting caught and having to pay thousands of dollars as a result is much higher than they likely believe.
The information provided here is general in nature, is not legal advice, and does not create an attorney-client relationship with Will Montague or Montague Law PLLC.
Yes, it is illegal, and yes, your friends should worry. Bit Torrent servers log the IP addresses of everyone participating in that torrent. The name of the owner of that IP address can be subpoenaed and then sued. Settlements are in the $3000 to $7500 range.
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Your friend is a FOOL. Google "copyright troll", Ira Siegel, John Steele, Paul Duffy, Copyright Enforcement Group, Prenda Law, and see what you find. The copyright trolls have tracker software that pegs the IP Address of illegal downloaders, and subpoenas and suits may well follow, costing you lots of hassles and expense. Torrents are now one of the WORST ways to file share. Do it much, particularly with porn, and you will be sorry, very sorry.
So far, this is free to you. Until you pay a fee, I am not your lawyer and you are not my client, so you take any free advice at your sole risk. I am licensed in IL, MO, TX and am a Reg. Pat. Atty. so advice in any other jurisdiction is general advice and should be confirmed with an attorney licensed in that jurisdiction.