I've managed to get LISE to function correctly with the new version of FEU by
doing the following...

/lib/fielddefs/FEUSingleSelect/lisefd.FEUSingleSelect.php

within "public function GetFEUUserList()"

Change 
    $tmpusers = $FEU->GetUsersInGroup($group);
to
    $groupname = $FEU->GetGroupName($group);
    $tmpusers = feu_smarty::get_users_by_groupname($groupname); 

within "public function RenderForAdminListing($id, $returnid)"

Change
   $user = $FEU->GetUserName($value[0]);
to
   $user = feu_smarty::get_username($value[0]); 

NOTE: I have noticed though, "username" as a FEU Field isn't possible without
further investigation as it produces a ID rather than a fieldname for the
"GETFEUUserList" function, but if you leave it blank, the system uses username
anyway! Also this only makes the "FEUSingleSelect" fielddef work.