Cheating is about doing something 'wrong' intentionally (wrong according to the personal standards and morals of a person). At least in my book. Generally I would say that as long as she wasn't doing something intentionally, like cultivating this relationship despite knowing that this would go against something you two agreed to (meaning: some agreements you two made in regard to your relationship), she shouldn't think about calling it cheating. The biggest indication for this would be that she found out about her feelings AFTER he broke up the contact with her.
What seems to make it hard for her is the general incompatibility of loving someone else if there is a love in her life already. Therefore, from her 'mono-mindset' it counts as cheating that she has had feelings for another man. Judging things doesn't have anything to do with how another person sees them. She judges her actions according to her inner moral standards and it may help her that you aren't putting some more pressure on her, but you won't dissolve this dilemma for her by saying "This is different for me." I have felt the same when I discovered my love for another man besides my husband and I needed years to overcome the thought of "This can't be right, what the heck is wrong with you?!"
Give her some time to sort her feelings out. If she never thought about the possibility of loving more than one this can be quite a shock for her.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.