Firstly, there is absolutely nothing in the scene that fits the definition (which has no clear parameters). Okay, we admit that there are people who consider even a glance towards other men to be cheating.
So, if it has no clear parameters, you can't say it doesn't fit the definition...

You're right, the question is settled, because the story itself considers the actions a 'betrayal', ie cheating, to some degree.
Otherwise, Helen wouldn't have felt guilty and Kate wouldn't have felt the need to rationalize, it wasn't a 'real' betrayal...
Helen hadn't just met James, she considered him her boyfriend... Her actions were wrong, and she (the dev and story), know it.
I will concede it might not be enough to warrant a cheating tag on this site, but Helen's actions were most definitely cheating. If it wasn't, it wouldn't have needed to be kept secret from James, another clue it was cheating. The fact it was 20 years ago, no continuation, is irrelevant.
Story wise, the only relevant fact is whether James knows about it and/or is 'ok' about what happened...
If he doesn't know (which appears he doesn't)...well...we'll have to wait and see...