You're assuming Ben needed her to go back at the time--Ben's acting like he cares about getting people back on the island, but there's reason to think that's no longer his major motivation, whereas it does seem to be his worst enemy's at the moment. If he's just after Penny or Widmore, he may have wanted her for some other reason. There was that unexplained violent altercation that marked him up, maybe related to what Kate refused to tell Jack about.

The music in the Ben-John scene strongly implied Ben's attention was wholly absorbed in the ring/Jin issue, and that his string-twangy sinister turn came then.