Just let her know that another area of the art has captured your interest and you would like the time/space to explore it.
Maybe suggest you explore it together (if she has the time/inclination)? This shouldn't end you friendship (still make time for her after you leave to study this other form).
If she's a real friend (and caring instructor) - should support and encourage you is all your dance endeavers...

Speaking from a teachers' POV, she may not be as upset as you imagine.
If you're really good friends, you should be able to be frank with her & not have it be an issue between you but rather a new topic of conversation.

On the other hand, maybe she views your all's 'friendship' differently than you think, that is, she's reserved enough professional distance to take any decision from a student in stride. Sometimes I think this happens with my students, they are reluctant to tell me something because they don't want to hurt my feelings or whatever, but actually I'm pretty detached from what they do.

I'll certainly voice my opinion (professional opinion OR personal opinion!) if asked (sometimes if not!) but whatever it is, it's not going to be an issue for me in the long run.

I just had two students move towards something they wanted to do that was not where my class was leading.

Its not a personal thing and I encouraged them to seek something new.

I view teaching as a process of personal growth for everyone involved. If I can be a stepping stone for something else, Im glad to be part of the path.

It's nice that you are concerned for your teacher/friend's feelings. Peoples interests change, or life changes their circumstances sometimes. She may already know that your interests have changed, and may understand and support your decision. Or, you may be parting ways temporarily.