Your friend's ISP (or other email provider) may well have blacklisted your IP address (or even your ISP) as a potential source of spam. It's a remarkably common experience.
Ask your friend (via your son, if necessary) to access her email via the web (rather than by using an email client, such as Outlook Express or Windows Live Mail). She should then look for a folder labeled 'Spam', 'Junk', 'Bulk' or something similar. Your mail may well have ended up in there. (If so, there should be an option to mark it as 'Not spam' - or something similar - so that future mail from you will get through).
Also try changing your IP address. (That usually involves nothing more than unplugging your router, waiting a few seconds and then reconnecting it). Check your current IP address here:
http://whatismyipaddress.com/
Then check again after disconnecting and reconnecting. (If only your IP address is blocked, rather than your ISP or other email service, you should then be able to contact your friend again).
Chris