I think it means she feels cheated that fate has seperated Shuin & Lenne.
Often, when a loved one is seperated from you, your first reaction would be, "How COULD you? How could you let this happen?" , etc. (the usual denial, anger, frustration, blah blah blah...)

I don't think he cheated. I think Lenne feels cheated or feels that it's unfair that she was seperated from her love.