I know this is difficult and uncomfortable, but dealing with it ahead of time is much better than trying to handle it diplomatically in front of the class-the person being dealt with usually gets "huffy" or becomes a human steamroller to get their way no matter what. It's important to contact this woman and settle things before the class. On her side she wants her friend to become a quilting friend-maybe just because they're friends or she knows her friend needs a creative activity to deal with grief, depression-whatever, she's convinced her friend to take the class because she'll be with her, and will feel she can't back down from that. So, if she balks at leaving and returning at the end of class I'd like to offer an alternative conversation. Because this has happened with poor results in the past, use those incidents in general terms and bring her around to how the situation needs to proceed: "Mary, because we're friends, and you're an experienced quilter, I know I can be honest with you and you'll understand". Then relate the pitfalls of past experiences: the non paying guest, in trying to help their friend/relative, ends up instructing over you, or offering alternative methods, which is confusing to newbies & also makes the others in the class uncomfortable. Because of complaints from those "others" (always good to have a nameless, faceless group that has brought about policy changes) you've had to alter your approach to these "situations" (you are not singling her & her friend out). After consulting other instructors you have adopted a reduced class fee for the more experienced quilter who wants to get a friend started, but they must participate and make the project as you are teaching it. This way they can "wordlessly" (stress that word) help their friend by example, and if they want to share things that they think might make it easier for their friend, this can be done one on one at another time, giving them additional opportunity to quilt together. Wind it up with your appreciation of her skill & talent (if this is true, or some type of positive comments) and the fact that she wants her friend to experience the joy of developing a creative skill. Also, you know you don't have to worry about her being a distraction in the class (hint,hint) and you appreciate her understanding the class policies you are having to adopt. Wow, sorry this is so long!