What's being alleged is that she deliberately escorted the man out through an exit that is not usually made available to members of the public, instead of allowing him to leave through the regular door that would likely have put him right into the hands of ICE. If that was done with the intent of helping him evade arrest (which, if the story above is accurate, seems likely), it seems very reasonable to charge her with obstruction.

None of that is to say that what she did was morally wrong—often the law and morality are at odds.