If she’s doing things you would only do with a partner AND feels the need to hide it - it’s cheating. End of story.
Any ‘normal’ set up, I would say walk right away - you already had one row but she carried on. She had her chance. However, you do have an unusual set up with the ‘open’ relationship so I would at least have a final conversation to fully confirm there were definitely no crossed wires. It doesn’t sound like there are though, given the previous argument.
Personally, regardless of the ‘open relationship’ (personally I don’t get this at all but understand it works for some people), exes being involved makes me really uncomfortable here… Or if it was a friend/someone you know. (This is also my opinion on threesomes). It just feels like anything like that has the extra potential to get messy.
(I realised after writing all this that this is an old thread that got bumped - oops. How did it go @Welsh-Warrior ?)