Recently that a law has come out about the wireless router.?:
The Computer Misuse Act, Cap 50A, is very clear that if one uses another person's computer service (in this case, someone's broadband connection) without permission, he has committed an offence.

I am unsure of the Act's legislation history but the law was around since 1993. From your post, I would assume that your friends made use of the other party's router fairly recently. As such, if they are caught, they can be prosecuted.

Arguing that the router is unsecured and/or does not have a unique SSID is futile; it is common sense that the router has to belong to somebody and if you are not in a free wireless zone e.g. McDonald's, then one should know that any access would probably be going through a privately owned network.