Because some people fraudulently complain to manufacturers and stores about products which had nothing wrong with them (or which they never even bought), seeking to get freebies, those manufacturers and stores keep records of all complaints and often share their databases.
So your friend's email address will have been entered into such a database to check that she wasn't, for example, making hundreds of complaints every month. Simply entering '
[email protected]' will then have shown that that email address was used by Mrs Agnes Bloggs when she made a complaint to (say) Asda last year, resulting in Asda asking for her postal address so that they could compensate her for her loss. Her postal address will then have been added to the database.