There are a couple of ways you can do this. The easiest is probably to just duplicate the email code and use the email address that the user entered like this:
$RecipientEmail = "".($WAGLOBAL_Contact_Email_To) ."";include("WA_Universal_Email/WAUE_contact_1.php");
$RecipientEmail = "".(isset($_POST["Email_address"]))?$_POST["Email_address"]:"") ."";include("WA_Universal_Email/WAUE_contact_1.php");
This will just send a duplicate of the message to the user. Another more complex way would be to duplicate the UE code then update the values to use a copy of the email page. You can then go into the copy of the email template and change the body to whatever you would like. This is a more advanced technique that requires coding and editing.
The other way to do this is to just use Universal Email to add a new server behavior to the contact page. You can just craft the message to be whatever you would like and send it to the posted email address:
isset($_POST["Email_address"])?$_POST["Email_address"]:""