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.

Can is generally used for present tense .It means the person can do it but it is not for sure that person will definitely do. COULD is used to denote past tense means work had been done earlier. May is used where the fact is possible or factual. Might is used where there is less possibilities.