Jenna,
While I have a strong feeling that these ladies are right that the "cat is out of the bag," and I also assume that you did not successfully convince your mom's friend that you were presenting as an actress in a local theatre production and had to get comfortable in public in the female role,

there is another feasible outcome.
It may be that your mom's friend will say nothing to your mom, thinking it may be best to say nothing to your mom until your mom decides to openly talk about her new daughter. In fact, that may be the kinder course of action for her to take. What your mom's friend does do or does not do is dependent on her personality and her relationship with your mom.
She may in fact talk about you with her husband or someone else to obtain advice about how she should proceed. We do not know what she will do, do we? I do not think that worrying is productive though, so try to avoid that. Easier said than done, I know. Instead, be wise.
In any event, I would think it would be a good idea to very soon bring up your trans status in an open conversation with your mom. You mentioned that you will do that in your more recent post.
She may be more understanding than you might think. Remember that her original reaction might be confusion and bewilderment, not anger, but also remember that she loves you.
It is good to be willing to go to gender therapy and to potentially have some sessions with your mom. Seeking out support groups for her, as well as you, could be a good thing too.
Hope for the best and I am wishing you the very best of results.
Chrissy