Is the wordpress user-model changed in wpmu?

The plugin should use the API, not some made-up SQL. addslashes() is not safe enough anyway.

$user = get_user_by( 'email', '[email protected]' );
echo $user->ID;

Besides that, the user tables are almost the same, there are just some tighter restrictions for user names in multi-site, because new sub domains could be made from the names.