You ended the friendship for a reason, you mentioned it was great to catch up with other friends and that you were sorry that you stayed away. So it's sounding like 'the friend' was feeling a little apprehensive over how the friendship ended and how you were explaining your absence from the group, perhaps. So she jumped into your seat after you left your seat to ensure her standing in the group hadn't changed.
By her behaviour she knows she wasn't at her best with you and now is ensuring there is no fallout within the wider group of friends, while jumping into your seat when it's empty is saying 'I'm not sorry'. She was probably hoping you'd make a little scene about sitting in her chair, that she could turn around to the group and let them know just how hard your treating her.
So ignoring the silent jab was great, keep having a good time despite her being there was golden. It effectively cut off any rude jab that she may have wished to make regarding how your treating her. Your decision to end your friendship with her obviously hit the mark, and the way you rose above her behaviour at the first group gathering shows a level of maturity that rose above her behaviour. If she keeps it up at the next gathering, then it will become obvious to others around you why you ended the friendship.