Two problems:
1) She wouldn't believe anyone could be happy as part of the hivemind, so she won't accept it
2) Even assuming she could accept that claim, what proof could they offer that the person isn't connected any more? They're unlikely to do it on her terms, so there's no guarantee they aren't playing tricks.
Besides, they didn't say they could remove someone. They only tacitly admitted that the effect could be reversed or cancelled. Combined with everything else the show revealed, the implication is that they're either psychologically incapable or unwilling to disconnect individuals from their hivemind, but they know it can still be done by an external actor. It would be like cutting off a limb.