You will not be able to invoke Outlook from a web application.
Firstly, you cannot invoke arbitrary processes on the client machine.
Secondly, you can't do this from the server machine, as it probably won't have Outlook, even if it does, it won't be able to use it as you describe.
Your best bet is probably sending a template to the user so they can customise and send it on.
Alternatively, you could send the message from the server and allow the user to insert their own text, sending the user a cc for reference purposes.
Mark