Since you’re so close, you may feel comfortable telling them that you fully support her decision and want to support her in ways that work for her. It may just be hanging around with her and being there for her in a variety of situations.
We have a friend who transitioned a little over a year ago. Her biggest challenge was transitioning in the work place where she had worked for years. She said it would have been easier changing jobs, but her entire career/seniority could be jeopardized.
Just be a good friend.
We go on outings together, do church stuff together, we help her with her transgender outreach initiatives that she has been working on, etc.