Give her the chance. What will it hurt if she actually shoots the big guy? As you say she has never killed a deer so why not let her see and have a chance of what we all look for when hunting. Do you want her as a hunting companion or just someone to tell your stories to? Part of the sport is allowing others to experience all they can and help them get the animal that will make them happy. I know it is bigger than any you have shot BUT why not set some priorities in your hunting goals. Would it be wrong to not let her have a chance at him? NO Would it be heplful with the relationship you two will have as hunting partners. Only you can answer that one.
I would give her the opportunity.
Let us know what she shoots.