Most states would prevent him from residing with children who are not his own. Before allowing him to live with her, she needs to consult with his supervising or registering officer (sex offender registry officer). This person will be familiar with the specific restrictions he has... and he does have restrictions. You are not on the registry if you have no restrictions.
Have her speak with his registering officer and also consult with a local attorney (even over the phone... shouldn't cost anything). She needs to be assured that allowing him in the home will not 1) violate his release conditions or 2) subject her to a criminal violation for facilitating some sort of registry violation.
I can tell you that in my jurisdiction if someone permits a registered sex offender to live in their residence with their minor children and the children are not the biological children of the sex offender, they will find themselves charged with the same criminal sexual offender registry violation as the person who is on the registry.
Since she could potentially be exposed to criminal sanctions in these situations, make sure she checks with a local attorney and his supervising officer to make sure there are not restrictions.
I am not your attorney. This forum is for general questions and answers and I cannot provide you with applicable law or legal opinions or advice in your jurisdiction. Please seek the advice of a competent attorney in your jurisdiction. While I can answer general questions and discuss general legal principles, I cannot give you legal advice in your state. The internet is a great place to get general information but should not be seen as a substitute for the attorney-client relationship or the services of a good lawyer in your state.