She can = She has the ability. It says nothing about her willingness.
She could = She has the ability, and it is possible that she will.
She may = It is possible she will. She may choose to help you. It doesn't explicitly say that she is able, but it is usually implied. May is a more old-fashioned version of might.
She might = very similar to she may, but I think this version is much more common now.