As Boxtops indicates, it's essential to get people to agree to being on a mailing list, otherwise it's ILLEGAL to send them unsolicited emails:
http://www.cognique.co.uk/blog/email-marketing-in-the-uk-legal-and-best-practice/
Using any email client (such as Outlook, Outlook Express, Windows Live Mail or Thunderbird) it's very easy to create mailing groups. Then, instead of entering a single name into the 'Send' field, you simply enter the name of the group and the mail is sent to everyone within that group.
A word of caution though:
Many ISPs limit the number of recipients that you can send a single email message to via their servers. (That's an anti-spamming measure). Some ISPs let you send to 50 or 100 people at time, whereas some have a limit as low as 20 recipients. So your friend might need to split a large number of intended recipients into several groups and then email each group separately in order to prevent the messages being blocked.
One way to collect contact details is to run a competition whereby customers are invited to drop their business cards into a receptacle to qualify for a draw. (A free meal for two or a bottle of bubbly are common prizes for such promotions). Blank cards can be made available for completion by those who've not got business cards and, of course, there needs to be a notice stating that the contact details provided may be used to send out details of promotions.
Chris