Legal Question in Family Law in Illinois

You asked the same question at the other web sit. The answer does not change. Illinois abhors common law marriage. The IL Supreme Court has spoken on the subject. One might argue a partnership agreement, but it is very hard to prove. If the person not in title did something that might entitle them to be compensated, it might be a different story, such as care for the dying partner, or working for the partner and not being compensated because they sought they were a partner. Otherwise, they are out of luck.