As was brought out, the marrying a non-JW (I can't bear to type "outside the truth") is not a judicial matter. The fact that the letter was from 2 elders and not 3 means this is an investigative committee not a judicial one. It sounds like Mom is pushing and now the BOE feels like they at least need to attempt to determine if some JW rule has been violated. Unless they have 2 witnesses to some alleged wrongdoing, they can't move forward. Of course, if she "bites" and confesses to something, then they could go after her judicially if they so choose. The rules say they don't HAVE to but they can. BTW - if they already had 2 witnesses, they likely would have just invited her to a JC. It sounds like they don't have enough to proceed.
I have a feeling that the BOE would like nothing better than to have this person just ignore them so they can say they tried. They don't even know her? I can tell you that those elders likely don't want to met with her anymore than she wants to met with them. She should tell them she knows where the KH is and how to find them if she ever decides to become active again. Then, she should enjoy her life with her new husband!