You won't find the electoral roll available online for free. Some companies might offer limited free search facilities but, since they pay a large amount of money to buy the data, they aim to make a profit.
However, most reference libraries will have the latest version of UK Info-Disk (which includes both the electoral roll and the phone directory) available for free consultation. You need to be aware, though, of the following:
(i) a few libraries (e.g. Camden) don't stock the Info-Disk because they say (incorrectly) that they'd be breaching data protection laws by making it available to library users;
(ii) the electoral roll which is on the disk (or available, for a fee, online) is only the 'edited' roll. Most people now indicate, when they register on the electoral roll, that they don't wish their name to appear on the edited roll (because it can be purchased by anyone, including those who compile mailing lists for junk mail) ;
(iii) the full electoral roll is only available, electronically, to authorised users, such as the police. Other people may consult the full roll at their local council offices but, obviously, this won't help you if you don't know which area your friend lives in ; and
(iv) if your friend hasn't moved for several years you might be lucky and find a library which has got an older version of the Info-Disk (from the years when the disk contained the full roll). Ipswich reference library has such a copy and I've persuaded them to load the software onto one of their PCs, so that library users may consult it.
Chris