Do you think Frederick was out to ruin James' hopes? Or did he just figure he'd have a good time with a willing girl and then she'd just go ahead and marry James, anyway? I'm fairly certain that he never expected to marry Isabella, and may have been willing to dally with her precisely because she was engaged already (a la Henry Crawford). I don't know if it occurred to him that she might have expectations of him, and I don't think he was thinking of James at all.