I'm afraid the javascript on your website can't access the source code of other websites due to the Same Origin Policy. This is to prevent Cross Site Scripting attacks. For example, if the user was logged into their email account, the Same Origin Policy prevents other websites, open in the same browser, from trying to access your email.
Your best bet is to:
- Send the url to your application server via AJAX
- Perform a HTTP GET from your application server (in Java / PHP etc)
- Reply to the AJAX request with the text of the other website
- Then show the source code to your user
