First it sounds like your mother is a smart lady to carry a big stick with her.
As any frequent walker/runner knows when you are out there on the road you are quite a target to loose dogs.
Just to clarify has Serena been to group obedience classes previously? Was it puppyK or beginners obedience?
I have this issue with my dog ( he is over 3 and about 104 lbs.) we let his wanting to visit other dogs get out of hand and now we must deal with it. Our goal at this point is for him to politely ignore other dogs when told to do so. It is slowly happening and sit/stay, and "enough" (when whining or barking) are wonderful useful commands. You could also teach "watch me" to help keep her focus on you rather than the other dog.
Whatever you decide I don't see any harm at all in taking a group class with the dog. I definitely think it will help you and her to achieve more control around other dogs. It sounds like Mom would like to watch and sit in on the classes, but maybe you need to handle her during the class to help you gain more control. It is so wonderful to have an objective view on the situation, and it has helped me greatly to have constructive criticism/feedback from attending my class.