You're absolutely right that your husband's friendship with this girl may be harming her more than it is helping her. Given her sexual abuse history, turning to a male who she may be attracted to in order to help her with these issues sounds totally counter-productive (and potentially harmful). She needs help from a trained therapist in order to process and deal with her difficult issues. Help from someone without the proper training (despite their good intentions)may very well make her problems worse-- not to mention the fact that they seem to harming your marriage with your husband.

Can you explain to him that his involvement may actually harm this girl? That the best thing he can do to help her is to support her in finding a qualified therapist?