As a transsexual parent of a transman myself, I would say; accept that your mother is an adult with her own story to tell.
It is really hard for even adult children envisaging their parents doing anything remotely connected to sexuality in any way, but you will just have to get over it and move on.
That sounds a bit harsh, but that is basically what it comes down to.
You will have to deal with a lot of Internalised Transphobic feelings you don't even know you have, but you can do it.
Please spend a lot of time talking to her on a deep and totally honest level, and you will gain a huge amount, and become even closer.
It's probably worthwhile getting here to join here for support too, regardless of what she eventually decides. The first step, as for all of us is to find a good Gender Therapist to help her ask herself the right questions and find answers.
I would be happy to spend time chatting to her anytime.
Karen.