Why not suggest that you and your Mom spend some time around the barn first and see how she feels? It would be good if there would be someone there, other than you, who could give her some refresher tips and tips on English style riding. It's really not ALL that different except with the rein contact. I would avoid being the one to give her advice on riding.
Then give it a try and don't expect too much the first few rides. Once you develop a kind of peace on the trails, you will be able to talk to her more about how you would like to ride and how often. If you don't push her away, this could actually end up one of two ways -- 1. She decides she loves this riding and wants to go as much as you do or 2. She sees how much you love it and are willing to be responsible and careful about it and will let you ride alone.
Or, of course, it could all go sour and she'll never let you ride again, but it doesn't really sound like that would happen. She seems like a supportive mom.