I hope Nina is indeed a friend and there's not some later twist where she turns out to be a plant from the ex to keep an eye on Rosalie or help keep her isolated or whatever.
Obviously there's no reason to suspect that right now, but given how hard this series has gone into the "literally everyone is against the main couple, either individually or jointly, for shallow, stupid, petty reasons just for the sake of evil lulz" realm so far I would not put it past them to end up using this to twist the knife a little more.