Two functions you must know when using arrays: array_keys and array_values. When using assoc arrays, you can split the keys and values up into separate non-assoc arrays.
This should do it.
$values = array(
'bob' => 'friend',
'sam' => 'enemy'
$isEnemyPresent = in_array('enemy', array_values($values));
Of course, if the value you are looking for is nested even deeper, it would be wiser to create a recursive method to dig into it.