Webform E-mail send to selected user
I believe hook_node_presave() along with user_load() is your answer

something like this:

function hook_node_presave($node) {
if($node-&gt;type == 'webform') {

$uid = user_load($node-&gt;FIELDOFUID);

$uid-&gt;mail;

// do your mail send here with $uid-&gt;mail;

}

}