Please contact a family law attorney such as myself that can aid you in preparing a case. Unlike most television programs where fault must be proved in order to get a divorce in Illinois fault is usually not assigned and plays little into the final settlement and resolution of the disillusion of the marriage. In Illinois the most common grounds are irreconcilable differences this provides the cleanest method for divorce. A private investigator can provide you proof of adultery but has little consequence as to the final settlement of the case. In other words don't waste your time or money hiring an attorney. Best of luck to you.
Unfortunately, in the small space provided for questions one is unable to get a full background of the matter. In the situations such as you are explaining you should immediately seek the counsel of a capable attorney that practices in this area of law or if you cannot afford an attorney, you should seek a social service organization in your area. This answer is general in nature and does not form an attorney client relationship. If you would like to get more information on this topic or to establish an attorney client relationship please contacts my office.
This may be deemed an advertisement and as such: information contained within this answer has been approved by Bradley L. Schencker, a licensed attorney to practice law in the State of Illinois, for any questions contact Bradley L. Schencker by the methods provided herein.