I assume that you are talking about the fictional story of Barbara Allen that has been made famous by folk songs and such. A version of that is in the link below.
If this is the whole story (and I have not heard or seen any versions of the song that differ greatly from this) then I would say her treatment of him is partly cruel and partly not.
Her original attitude towards him is not cruel. If he really loved her so much (that he would die because of her scorn) then why did he "slight" her when making toasts? In addition, it's not her fault that he had it so bad for her that he would waste away because she's mad at him.
However, once she comes to see him, she might as well have been a little nicer. Sure, Jemmy said mean things to her then, but he's dying -- why not be a bit nicer to him. She could have said she was sorry even if she wasn't. She could have shown some compassion instead of basically saying "you're dying and that's what you get for being rude to me."
So I don't think that she was cruel to "make" him pine away for love of her. But I do think it was a bit cruel for her to act as she did as he lay dying.