Is her son entitled to any of his mother's estate?

If she chooses to do nothing then your example is correct. However, she can make an estate plan. She/they should consider a trust that may protect both her children and his children. However, if an estate is small ($100,000 or so) and there is a surviving spouse, even if she has a Will, it may still all go to husband. She should speak with an attorney regarding her planning options. This is opinion is solely based upon the facts presented in the inquiry. Additional facts may be important and may change the analysis. If you are uncertain, seek legal counsel. We are not your attorneys. This answer is being offered to assist you in determining if you need to retain legal counsel to assist you, not to resolve your issue through an email inquiry.