The term "cheating' is relative, as far as I'm concerned - everyone can give their own opinion about what they consider cheating, but only two opinions matter - yours and your partner's.
If you think of cheating in a game, it means "not following the rules". To me, cheating in a relationship is exactly that.
The only difference between monogamous and polyamorous relationships is that the rules are different. To a monogamous person, having sex or a relationship with another, with or without permission is automatically cheating, to a poly person, if it's done with everyone's consent, normally it isn't. For some engaging in a BDSM activity with a third is cheating, for others it isn't.
One of the problems that we have in society, in my opinion, is that there are these generally-acknowledged "rules" for relationships - only problem is, different people interpret them differently. Some people regard their husbands going to a strip club as cheating, or talking to a woman on the internet (extreme examples, maybe, but I've seen them).
The important thing is that you are your partner feel free to work out what your "rules" are, and be prepared to review them, according to each of your needs. Once you have those in place, then you both know exactly what is and isn't cheating in your relationship. This is one of the key parts of defining a relationship, as far as i am concerned.
I caution you to treat anyone else who provides specifics as to what THEY regard as cheating as input to your own process, not as a "rule-of-thumb" to stick to.
So if your partner thought she was breaking the rules of what she understood to be the relationship, then she is entitled to that. Maybe that should be a prompt for the two of you to sit down and agree on what those rules should be....
"Listen, or your tongue will make you deaf." - Native American Proverb